class Program
{
static void Main(string[] args)
{
var c = new SampleClass<ClassString>();
c.ClassStrings.Add(new ClassString{ Name1 = "1", Name2 = "1"});
c.ClassStrings.Add(new ClassString{ Name1 = "2", Name2 = "2"});
var result = c.Query<ClassString>().Where(s => s.Name1.Equals("2"));
Console.WriteLine(result);
Console.ReadLine();
}
}
public class ClassString
{
public string Name1 { get; set; }
public string Name2 { get; set; }
}
public interface ISampleQ
{
IQueryable<T> Query<T>() where T: class , new();
}
public class SampleClass<X> : ISampleQ
{
public List<X> ClassStrings { get; private set; }
public SampleClass()
{
ClassStrings = new List<X>();
}
public IQueryable<T> Query<T>() where T : class, new()
{
//Get the WHERE expression from here.
return new EnumerableQuery<T>((IEnumerable<T>) ClassStrings);
}
}
I looked into this solution1, solution2 and solution3 seems not applicable to my question. Since the where clause was defined outside and it was interface of the class. How to get the expression inside Query method? since no variable being pass thru.
The purpose, I want the retrieve and injected back into the destination (which is DBContext as IQueryable). Because we have a common interface for us like this ISampleQ.
Added new sample codes but same scenario:
internal class Program
{
private static void Main(string[] args)
{
var oracleDbContext = new OracleDbContext();
var result = oracleDbContext.Query<Person>().Where(person => person.Name.Equals("username"));
Console.WriteLine();
Console.ReadLine();
}
}
public interface IGenericQuery
{
IQueryable<T> Query<T>() where T : class , new();
}
public class OracleDbContext : IGenericQuery
{
public OracleDbContext()
{
//Will hold all oracle operations here. For brevity, only
//Query are exposed.
}
public IQueryable<T> Query<T>() where T : class, new()
{
//Get the where predicate here. Since the where was defined outside of the
//class. I want to retrieve since the IQueryable<T> is generic to both class
//OracleDbContext and MssqlDbContext. I want to re-inject the where or add
//new expression before calling.
//
//For eg.
//oracleDbContext.Query<T>(where clause from here)
return null;
}
}
public class MssqlDbContext : IGenericQuery
{
public MssqlDbContext()
{
//Will hold all MSSQL operations here. For brevity, only
//Query are exposed.
}
public IQueryable<T> Query<T>() where T : class, new()
{
//Get the where predicate here.
return null;
}
}
public class Person
{
public int Id { get; set; }
public int Name { get; set; }
}
It is quite complex... Now... the Queryable.Where() works this way:
public static IQueryable<TSource> Where<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, bool>> predicate)
{
return source.Provider.CreateQuery<TSource>(Expression.Call(null, ...
So the Queryable.Where calls the source.Provider.CreateQuery() that retuns a new IQueryable<>. So if you want to be able to "see" a Where() while it is being added (and manipulate it), you must "be" the IQueryable<>.Provider, and have your CreateQuery(), so you must create a class that implements IQueryProvider (and probably a class that implements IQueryable<T>).
Another way (much simpler) is to have a simple query "converter": a method that accepts a IQueryable<> and returns a manipulated IQueryable<>:
var result = c.Query<ClassString>().Where(s => s.Name1.Equals("2")).FixMyQuery();
As I said, the full route is quite long:
namespace Utilities
{
using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Data.Entity;
using System.Data.Entity.Infrastructure;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;
using System.Threading;
using System.Threading.Tasks;
public class ProxyDbContext : DbContext
{
protected static readonly MethodInfo ProxifySetsMethod = typeof(ProxyDbContext).GetMethod("ProxifySets", BindingFlags.Instance | BindingFlags.NonPublic);
protected static class ProxyDbContexSetter<TContext> where TContext : ProxyDbContext
{
public static readonly Action<TContext> Do = x => { };
static ProxyDbContexSetter()
{
var properties = typeof(TContext).GetProperties(BindingFlags.Instance | BindingFlags.Public | BindingFlags.FlattenHierarchy);
ParameterExpression context = Expression.Parameter(typeof(TContext), "context");
FieldInfo manipulatorField = typeof(ProxyDbContext).GetField("Manipulator", BindingFlags.Instance | BindingFlags.Public);
Expression manipulator = Expression.Field(context, manipulatorField);
var sets = new List<Expression>();
foreach (PropertyInfo property in properties)
{
if (property.GetMethod == null)
{
continue;
}
MethodInfo setMethod = property.SetMethod;
if (setMethod != null && !setMethod.IsPublic)
{
continue;
}
Type type = property.PropertyType;
Type entityType = GetIDbSetTypeArgument(type);
if (entityType == null)
{
continue;
}
if (!type.IsAssignableFrom(typeof(DbSet<>).MakeGenericType(entityType)))
{
continue;
}
Type dbSetType = typeof(DbSet<>).MakeGenericType(entityType);
ConstructorInfo constructor = typeof(ProxyDbSet<>)
.MakeGenericType(entityType)
.GetConstructor(new[]
{
dbSetType,
typeof(Func<bool, Expression, Expression>)
});
MemberExpression property2 = Expression.Property(context, property);
BinaryExpression assign = Expression.Assign(property2, Expression.New(constructor, Expression.Convert(property2, dbSetType), manipulator));
sets.Add(assign);
}
Expression<Action<TContext>> lambda = Expression.Lambda<Action<TContext>>(Expression.Block(sets), context);
Do = lambda.Compile();
}
// Gets the T of IDbSetlt;T>
private static Type GetIDbSetTypeArgument(Type type)
{
IEnumerable<Type> interfaces = type.IsInterface ?
new[] { type }.Concat(type.GetInterfaces()) :
type.GetInterfaces();
Type argument = (from x in interfaces
where x.IsGenericType
let gt = x.GetGenericTypeDefinition()
where gt == typeof(IDbSet<>)
select x.GetGenericArguments()[0]).SingleOrDefault();
return argument;
}
}
public readonly Func<bool, Expression, Expression> Manipulator;
/// <summary>
///
/// </summary>
/// <param name="manipulator">First parameter: true for Execute, false for CreateQuery.</param>
/// <param name="resetSets">True to have all the DbSet<TEntity> and IDbSet<TEntity> proxified</param>
public ProxyDbContext(Func<bool, Expression, Expression> manipulator, bool resetSets = true)
{
Manipulator = manipulator;
if (resetSets)
{
ProxifySetsMethod.MakeGenericMethod(GetType()).Invoke(this, null);
}
}
/// <summary>
///
/// </summary>
/// <param name="nameOrConnectionString"></param>
/// <param name="manipulator">First parameter: true for Execute, false for CreateQuery.</param>
/// <param name="resetSets">True to have all the DbSet<TEntity> and IDbSet<TEntity> proxified</param>
public ProxyDbContext(string nameOrConnectionString, Func<bool, Expression, Expression> manipulator, bool resetSets = true)
: base(nameOrConnectionString)
{
Manipulator = manipulator;
if (resetSets)
{
ProxifySetsMethod.MakeGenericMethod(GetType()).Invoke(this, null);
}
}
protected void ProxifySets<TContext>() where TContext : ProxyDbContext
{
ProxyDbContexSetter<TContext>.Do((TContext)this);
}
public override DbSet<TEntity> Set<TEntity>()
{
return new ProxyDbSet<TEntity>(base.Set<TEntity>(), Manipulator);
}
public override DbSet Set(Type entityType)
{
DbSet set = base.Set(entityType);
ConstructorInfo constructor = typeof(ProxyDbSetNonGeneric<>)
.MakeGenericType(entityType)
.GetConstructor(new[]
{
typeof(DbSet),
typeof(Func<bool, Expression, Expression>)
});
return (DbSet)constructor.Invoke(new object[] { set, Manipulator });
}
}
/// <summary>
/// The DbSet, that is implemented as InternalDbSet<> by EF.
/// </summary>
/// <typeparam name="TEntity"></typeparam>
public class ProxyDbSetNonGeneric<TEntity> : DbSet, IQueryable<TEntity>, IEnumerable<TEntity>, IDbAsyncEnumerable<TEntity>, IQueryable, IEnumerable, IDbAsyncEnumerable where TEntity : class
{
protected readonly DbSet BaseDbSet;
protected readonly IQueryable<TEntity> ProxyQueryable;
public readonly Func<bool, Expression, Expression> Manipulator;
protected readonly FieldInfo InternalSetField = typeof(DbSet).GetField("_internalSet", BindingFlags.Instance | BindingFlags.NonPublic);
/// <summary>
///
/// </summary>
/// <param name="baseDbSet"></param>
/// <param name="manipulator">First parameter: true for Execute, false for CreateQuery.</param>
public ProxyDbSetNonGeneric(DbSet baseDbSet, Func<bool, Expression, Expression> manipulator)
{
BaseDbSet = baseDbSet;
IQueryProvider provider = ((IQueryable)baseDbSet).Provider;
ProxyDbProvider proxyDbProvider = new ProxyDbProvider(provider, manipulator);
ProxyQueryable = proxyDbProvider.CreateQuery<TEntity>(((IQueryable)baseDbSet).Expression);
Manipulator = manipulator;
if (InternalSetField != null)
{
InternalSetField.SetValue(this, InternalSetField.GetValue(baseDbSet));
}
}
/// <summary>
///
/// </summary>
/// <param name="baseDbSet"></param>
/// <param name="proxyQueryable"></param>
/// <param name="manipulator">First parameter: true for Execute, false for CreateQuery.</param>
public ProxyDbSetNonGeneric(DbSet baseDbSet, ProxyQueryable<TEntity> proxyQueryable, Func<bool, Expression, Expression> manipulator)
{
BaseDbSet = baseDbSet;
ProxyQueryable = proxyQueryable;
Manipulator = manipulator;
if (InternalSetField != null)
{
InternalSetField.SetValue(this, InternalSetField.GetValue(baseDbSet));
}
}
public override object Add(object entity)
{
return BaseDbSet.Add(entity);
}
public override IEnumerable AddRange(IEnumerable entities)
{
return BaseDbSet.AddRange(entities);
}
public override DbQuery AsNoTracking()
{
return new ProxyDbSetNonGeneric<TEntity>(BaseDbSet, new ProxyQueryable<TEntity>((ProxyDbProvider)ProxyQueryable.Provider, (IQueryable<TEntity>)BaseDbSet.AsNoTracking()), Manipulator);
}
[Obsolete]
public override DbQuery AsStreaming()
{
#pragma warning disable 618
return new ProxyDbSetNonGeneric<TEntity>(BaseDbSet, new ProxyQueryable<TEntity>((ProxyDbProvider)ProxyQueryable.Provider, (IQueryable<TEntity>)BaseDbSet.AsStreaming()), Manipulator);
#pragma warning restore 618
}
public override object Attach(object entity)
{
return BaseDbSet.Attach(entity);
}
public override object Create(Type derivedEntityType)
{
return BaseDbSet.Create(derivedEntityType);
}
public override object Create()
{
return BaseDbSet.Create();
}
public override object Find(params object[] keyValues)
{
return BaseDbSet.Find(keyValues);
}
public override Task<object> FindAsync(CancellationToken cancellationToken, params object[] keyValues)
{
return BaseDbSet.FindAsync(cancellationToken, keyValues);
}
public override Task<object> FindAsync(params object[] keyValues)
{
return BaseDbSet.FindAsync(keyValues);
}
public override DbQuery Include(string path)
{
return new ProxyDbSetNonGeneric<TEntity>(BaseDbSet, new ProxyQueryable<TEntity>((ProxyDbProvider)ProxyQueryable.Provider, (IQueryable<TEntity>)BaseDbSet.Include(path)), Manipulator);
}
public override IList Local
{
get
{
return BaseDbSet.Local;
}
}
public override object Remove(object entity)
{
return BaseDbSet.Remove(entity);
}
public override IEnumerable RemoveRange(IEnumerable entities)
{
return BaseDbSet.RemoveRange(entities);
}
public override DbSqlQuery SqlQuery(string sql, params object[] parameters)
{
return BaseDbSet.SqlQuery(sql, parameters);
}
IEnumerator<TEntity> IEnumerable<TEntity>.GetEnumerator()
{
return ProxyQueryable.GetEnumerator();
}
IEnumerator IEnumerable.GetEnumerator()
{
return ((IEnumerable)ProxyQueryable).GetEnumerator();
}
Type IQueryable.ElementType
{
get { return ProxyQueryable.ElementType; }
}
Expression IQueryable.Expression
{
get { return ProxyQueryable.Expression; }
}
IQueryProvider IQueryable.Provider
{
get { return ProxyQueryable.Provider; }
}
IDbAsyncEnumerator<TEntity> IDbAsyncEnumerable<TEntity>.GetAsyncEnumerator()
{
return ((IDbAsyncEnumerable<TEntity>)ProxyQueryable).GetAsyncEnumerator();
}
IDbAsyncEnumerator IDbAsyncEnumerable.GetAsyncEnumerator()
{
return ((IDbAsyncEnumerable)ProxyQueryable).GetAsyncEnumerator();
}
public override string ToString()
{
return ProxyQueryable.ToString();
}
}
public class ProxyDbSet<TEntity> : DbSet<TEntity>, IQueryable<TEntity>, IEnumerable<TEntity>, IDbAsyncEnumerable<TEntity>, IQueryable, IEnumerable, IDbAsyncEnumerable where TEntity : class
{
protected readonly DbSet<TEntity> BaseDbSet;
protected readonly IQueryable<TEntity> ProxyQueryable;
public readonly Func<bool, Expression, Expression> Manipulator;
protected readonly FieldInfo InternalSetField = typeof(DbSet<TEntity>).GetField("_internalSet", BindingFlags.Instance | BindingFlags.NonPublic);
/// <summary>
///
/// </summary>
/// <param name="baseDbSet"></param>
/// <param name="manipulator">First parameter: true for Execute, false for CreateQuery.</param>
public ProxyDbSet(DbSet<TEntity> baseDbSet, Func<bool, Expression, Expression> manipulator)
{
BaseDbSet = baseDbSet;
IQueryProvider provider = ((IQueryable)baseDbSet).Provider;
ProxyDbProvider proxyDbProvider = new ProxyDbProvider(provider, manipulator);
ProxyQueryable = proxyDbProvider.CreateQuery<TEntity>(((IQueryable)baseDbSet).Expression);
Manipulator = manipulator;
if (InternalSetField != null)
{
InternalSetField.SetValue(this, InternalSetField.GetValue(baseDbSet));
}
}
/// <summary>
///
/// </summary>
/// <param name="baseDbSet"></param>
/// <param name="proxyQueryable"></param>
/// <param name="manipulator">First parameter: true for Execute, false for CreateQuery.</param>
public ProxyDbSet(DbSet<TEntity> baseDbSet, ProxyQueryable<TEntity> proxyQueryable, Func<bool, Expression, Expression> manipulator)
{
BaseDbSet = baseDbSet;
ProxyQueryable = proxyQueryable;
Manipulator = manipulator;
if (InternalSetField != null)
{
InternalSetField.SetValue(this, InternalSetField.GetValue(baseDbSet));
}
}
public override TEntity Add(TEntity entity)
{
return BaseDbSet.Add(entity);
}
public override IEnumerable<TEntity> AddRange(IEnumerable<TEntity> entities)
{
return BaseDbSet.AddRange(entities);
}
public override DbQuery<TEntity> AsNoTracking()
{
return new ProxyDbSet<TEntity>(BaseDbSet, new ProxyQueryable<TEntity>((ProxyDbProvider)ProxyQueryable.Provider, BaseDbSet.AsNoTracking()), Manipulator);
}
[Obsolete]
public override DbQuery<TEntity> AsStreaming()
{
#pragma warning disable 618
return new ProxyDbSet<TEntity>(BaseDbSet, new ProxyQueryable<TEntity>((ProxyDbProvider)ProxyQueryable.Provider, BaseDbSet.AsStreaming()), Manipulator);
#pragma warning restore 618
}
public override TEntity Attach(TEntity entity)
{
return BaseDbSet.Attach(entity);
}
public override TDerivedEntity Create<TDerivedEntity>()
{
return BaseDbSet.Create<TDerivedEntity>();
}
public override TEntity Create()
{
return BaseDbSet.Create();
}
public override TEntity Find(params object[] keyValues)
{
return BaseDbSet.Find(keyValues);
}
public override Task<TEntity> FindAsync(CancellationToken cancellationToken, params object[] keyValues)
{
return BaseDbSet.FindAsync(cancellationToken, keyValues);
}
public override Task<TEntity> FindAsync(params object[] keyValues)
{
return BaseDbSet.FindAsync(keyValues);
}
public override DbQuery<TEntity> Include(string path)
{
return new ProxyDbSet<TEntity>(BaseDbSet, new ProxyQueryable<TEntity>((ProxyDbProvider)ProxyQueryable.Provider, BaseDbSet.Include(path)), Manipulator);
}
public override ObservableCollection<TEntity> Local
{
get
{
return BaseDbSet.Local;
}
}
public override TEntity Remove(TEntity entity)
{
return BaseDbSet.Remove(entity);
}
public override IEnumerable<TEntity> RemoveRange(IEnumerable<TEntity> entities)
{
return BaseDbSet.RemoveRange(entities);
}
public override DbSqlQuery<TEntity> SqlQuery(string sql, params object[] parameters)
{
return BaseDbSet.SqlQuery(sql, parameters);
}
IEnumerator<TEntity> IEnumerable<TEntity>.GetEnumerator()
{
return ProxyQueryable.GetEnumerator();
}
IEnumerator IEnumerable.GetEnumerator()
{
return ((IEnumerable)ProxyQueryable).GetEnumerator();
}
Type IQueryable.ElementType
{
get { return ProxyQueryable.ElementType; }
}
Expression IQueryable.Expression
{
get { return ProxyQueryable.Expression; }
}
IQueryProvider IQueryable.Provider
{
get { return ProxyQueryable.Provider; }
}
IDbAsyncEnumerator<TEntity> IDbAsyncEnumerable<TEntity>.GetAsyncEnumerator()
{
return ((IDbAsyncEnumerable<TEntity>)ProxyQueryable).GetAsyncEnumerator();
}
IDbAsyncEnumerator IDbAsyncEnumerable.GetAsyncEnumerator()
{
return ((IDbAsyncEnumerable)ProxyQueryable).GetAsyncEnumerator();
}
public override string ToString()
{
return ProxyQueryable.ToString();
}
// Note that the operator isn't virtual! If you do:
// DbSet<Foo> foo = new ProxyDbSet<Foo>(...)
// DbSet foo2 = (DbSet)foo;
// Then you'll have a non-proxed DbSet!
public static implicit operator ProxyDbSetNonGeneric<TEntity>(ProxyDbSet<TEntity> entry)
{
return new ProxyDbSetNonGeneric<TEntity>((DbSet)entry.BaseDbSet, entry.Manipulator);
}
}
public class ProxyDbProvider : IQueryProvider, IDbAsyncQueryProvider
{
protected readonly IQueryProvider BaseQueryProvider;
public readonly Func<bool, Expression, Expression> Manipulator;
/// <summary>
///
/// </summary>
/// <param name="baseQueryProvider"></param>
/// <param name="manipulator">First parameter: true for Execute, false for CreateQuery.</param>
public ProxyDbProvider(IQueryProvider baseQueryProvider, Func<bool, Expression, Expression> manipulator)
{
BaseQueryProvider = baseQueryProvider;
Manipulator = manipulator;
}
public IQueryable<TElement> CreateQuery<TElement>(Expression expression)
{
Expression expression2 = Manipulator != null ? Manipulator(false, expression) : expression;
IQueryable<TElement> query = BaseQueryProvider.CreateQuery<TElement>(expression2);
IQueryProvider provider = query.Provider;
ProxyDbProvider proxy = provider == BaseQueryProvider ? this : new ProxyDbProvider(provider, Manipulator);
return new ProxyQueryable<TElement>(proxy, query);
}
protected static readonly MethodInfo CreateQueryNonGenericToGenericMethod = typeof(ProxyDbProvider).GetMethod("CreateQueryNonGenericToGeneric", BindingFlags.Static | BindingFlags.NonPublic);
public IQueryable CreateQuery(Expression expression)
{
Expression expression2 = Manipulator != null ? Manipulator(false, expression) : expression;
IQueryable query = BaseQueryProvider.CreateQuery(expression2);
IQueryProvider provider = query.Provider;
ProxyDbProvider proxy = provider == BaseQueryProvider ? this : new ProxyDbProvider(provider, Manipulator);
Type entityType = GetIQueryableTypeArgument(query.GetType());
if (entityType == null)
{
return new ProxyQueryable(proxy, query);
}
else
{
return (IQueryable)CreateQueryNonGenericToGenericMethod.MakeGenericMethod(entityType).Invoke(null, new object[] { proxy, query });
}
}
protected static ProxyQueryable<TElement> CreateQueryNonGenericToGeneric<TElement>(ProxyDbProvider proxy, IQueryable<TElement> query)
{
return new ProxyQueryable<TElement>(proxy, query);
}
public TResult Execute<TResult>(Expression expression)
{
Expression expression2 = Manipulator != null ? Manipulator(true, expression) : expression;
return BaseQueryProvider.Execute<TResult>(expression2);
}
public object Execute(Expression expression)
{
Expression expression2 = Manipulator != null ? Manipulator(true, expression) : expression;
return BaseQueryProvider.Execute(expression2);
}
// Gets the T of IQueryablelt;T>
protected static Type GetIQueryableTypeArgument(Type type)
{
IEnumerable<Type> interfaces = type.IsInterface ?
new[] { type }.Concat(type.GetInterfaces()) :
type.GetInterfaces();
Type argument = (from x in interfaces
where x.IsGenericType
let gt = x.GetGenericTypeDefinition()
where gt == typeof(IQueryable<>)
select x.GetGenericArguments()[0]).FirstOrDefault();
return argument;
}
public Task<TResult> ExecuteAsync<TResult>(Expression expression, CancellationToken cancellationToken)
{
var asyncQueryProvider = BaseQueryProvider as IDbAsyncQueryProvider;
if (asyncQueryProvider == null)
{
throw new NotSupportedException();
}
Expression expression2 = Manipulator != null ? Manipulator(true, expression) : expression;
return asyncQueryProvider.ExecuteAsync<TResult>(expression2, cancellationToken);
}
public Task<object> ExecuteAsync(Expression expression, CancellationToken cancellationToken)
{
var asyncQueryProvider = BaseQueryProvider as IDbAsyncQueryProvider;
if (asyncQueryProvider == null)
{
throw new NotSupportedException();
}
Expression expression2 = Manipulator != null ? Manipulator(true, expression) : expression;
return asyncQueryProvider.ExecuteAsync(expression2, cancellationToken);
}
}
public class ProxyQueryable : IOrderedQueryable, IQueryable, IEnumerable, IDbAsyncEnumerable
{
protected readonly ProxyDbProvider ProxyDbProvider;
protected readonly IQueryable BaseQueryable;
public ProxyQueryable(ProxyDbProvider proxyDbProvider, IQueryable baseQueryable)
{
ProxyDbProvider = proxyDbProvider;
BaseQueryable = baseQueryable;
}
public IEnumerator GetEnumerator()
{
return BaseQueryable.GetEnumerator();
}
public Type ElementType
{
get { return BaseQueryable.ElementType; }
}
public Expression Expression
{
get { return BaseQueryable.Expression; }
}
public IQueryProvider Provider
{
get { return ProxyDbProvider; }
}
public override string ToString()
{
return BaseQueryable.ToString();
}
IDbAsyncEnumerator IDbAsyncEnumerable.GetAsyncEnumerator()
{
var asyncEnumerator = BaseQueryable as IDbAsyncEnumerable;
if (asyncEnumerator == null)
{
throw new NotSupportedException();
}
return asyncEnumerator.GetAsyncEnumerator();
}
}
public class ProxyQueryable<TElement> : IOrderedQueryable<TElement>, IQueryable<TElement>, IEnumerable<TElement>, IDbAsyncEnumerable<TElement>, IOrderedQueryable, IQueryable, IEnumerable, IDbAsyncEnumerable
{
protected readonly ProxyDbProvider ProxyDbProvider;
protected readonly IQueryable<TElement> BaseQueryable;
public ProxyQueryable(ProxyDbProvider proxyDbProvider, IQueryable<TElement> baseQueryable)
{
ProxyDbProvider = proxyDbProvider;
BaseQueryable = baseQueryable;
}
public IEnumerator<TElement> GetEnumerator()
{
return BaseQueryable.GetEnumerator();
}
IEnumerator IEnumerable.GetEnumerator()
{
return ((IEnumerable)BaseQueryable).GetEnumerator();
}
public Type ElementType
{
get { return BaseQueryable.ElementType; }
}
public Expression Expression
{
get { return BaseQueryable.Expression; }
}
public IQueryProvider Provider
{
get { return ProxyDbProvider; }
}
public override string ToString()
{
return BaseQueryable.ToString();
}
public IDbAsyncEnumerator<TElement> GetAsyncEnumerator()
{
var asyncEnumerator = BaseQueryable as IDbAsyncEnumerable<TElement>;
if (asyncEnumerator == null)
{
throw new NotSupportedException();
}
return asyncEnumerator.GetAsyncEnumerator();
}
IDbAsyncEnumerator IDbAsyncEnumerable.GetAsyncEnumerator()
{
var asyncEnumerator = BaseQueryable as IDbAsyncEnumerable;
if (asyncEnumerator == null)
{
throw new NotSupportedException();
}
return asyncEnumerator.GetAsyncEnumerator();
}
}
}
An example of a manipulator of Expressions (this one will transform .Where(x => something) to .Where(x => something && something):
namespace My
{
using System.Linq;
using System.Linq.Expressions;
public class MyExpressionManipulator : ExpressionVisitor
{
protected override Expression VisitMethodCall(MethodCallExpression node)
{
if (node.Method.DeclaringType == typeof(Queryable) && node.Method.Name == "Where" && node.Arguments.Count == 2)
{
// Transforms all the .Where(x => something) in
// .Where(x => something && something)
if (node.Arguments[1].NodeType == ExpressionType.Quote)
{
UnaryExpression argument1 = (UnaryExpression)node.Arguments[1]; // Expression.Quote
if (argument1.Operand.NodeType == ExpressionType.Lambda)
{
LambdaExpression argument1lambda = (LambdaExpression)argument1.Operand;
// Important: at each step you'll reevalute the
// full expression! Try to not replace twice
// the expression!
// So if you have a query like:
// var res = ctx.Where(x => true).Where(x => true).Select(x => 1)
// the first time you'll visit
// ctx.Where(x => true)
// and you'll obtain
// ctx.Where(x => true && true)
// the second time you'll visit
// ctx.Where(x => true && true).Where(x => true)
// and you want to obtain
// ctx.Where(x => true && true).Where(x => true && true)
// and not
// ctx.Where(x => (true && true) && (true && true)).Where(x => true && true)
if (argument1lambda.Body.NodeType != ExpressionType.AndAlso)
{
var arguments = new Expression[node.Arguments.Count];
node.Arguments.CopyTo(arguments, 0);
arguments[1] = Expression.Quote(Expression.Lambda(Expression.AndAlso(argument1lambda.Body, argument1lambda.Body), argument1lambda.Parameters));
MethodCallExpression node2 = Expression.Call(node.Object, node.Method, arguments);
node = node2;
}
}
}
}
return base.VisitMethodCall(node);
}
}
}
Now... How to use it? The best way is to derive your context (in this case Model1) not from DbContext but from ProxyDbContext, like this:
public partial class Model1 : ProxyDbContext
{
public Model1()
: base("name=Model1", Manipulate)
{
}
/// <summary>
///
/// </summary>
/// <param name="executing">true: the returned Expression will be executed directly, false: the returned expression will be returned as IQueryable<>.</param>
/// <param name="expression"></param>
/// <returns></returns>
private static Expression Manipulate(bool executing, Expression expression)
{
// See the annotation about reexecuting the same visitor
// multiple times in MyExpressionManipulator().Visit .
// By executing the visitor only on executing == true,
// and simply return expression; on executing == false,
// you have the guarantee that an expression won't be
// manipulated multiple times.
// As written now, the expression will be manipulated
// multiple times.
return new MyExpressionManipulator().Visit(expression);
}
// Some tables
public virtual DbSet<Parent> Parent { get; set; }
public virtual IDbSet<Child> Child { get; set; }
Then it is very transparent:
// Where Model1: class Model1 : ProxyDbContext {}
using (var ctx = new Model1())
{
// Your query
var res = ctx.Parent.Where(x => x.Id > 100);
// The query is automatically manipulated by your Manipulate method
}
another way to do this without subclassing from ProxyDbContext:
// Where Model1: class Model1 : ProxyDbContext {}
using (var ctx = new Model1())
{
Func<Expression, Expression> manipulator = new MyExpressionManipulator().Visit;
ctx.Parent = new ProxyDbSet<Parent>(ctx.Parent, manipulator);
ctx.Child = new ProxyDbSet<Child>(ctx.Child, manipulator);
// Your query
var res = ctx.Parent.Where(x => x.Id > 100);
}
The ProxyDbContext<> replaces the DbSet<>/IDbSet<> that are present in your context with some ProxyDbSet<>.
In the second example this action is done explicitly, but note taht you can create a method to do it, or create a factory for your context (a static method that returns a context with the various DbSet<> "proxied"), or you could put the proxification in the constructor of your context (because the "original" initialization of the DbSet<> happens in the constructor of DbContext, and the body of the constructor of your context is executed after this), or you could create multiple subclasses of your context, each one that has the constructor that proxifies in a different way...
Note that the first method (subclassing ProxyDbContext<>) "fixes" the Set<>/Set methods that otherwise you'll have to fix yourself by copying the code of the overloads of these two methodsd from ProxyDbContext<>.