I have a DataClassesDataContext containing a group of tables, and I am trying to do lambda expression filtering dynamically using only the name of the tables and the names of the fields. Basically I want to find for each table if a row with a specific ID already exists.
If I knew the table ahead of time, I would use :
if (dataClassesDataContext.MYTABLEXs.SingleOrDefault(m => m.MYTABLEX_ID == MyId))
DoExists();
But as I am getting tables names MYTABLEX and MYTABLEY (and fields names MYTABLEX_ID and MYTABLEY_ID) as strings on the fly, I am trying to build the above filter at runtime.
I can access the table dynamically using :
Type tableType = Type.GetType(incommingtableName); // incommingtableName being looped over MYTABLEX, MYTABLEY , ...
var dbTable = dataClassesDataContext.GetTable(tableType);
But then I am stuck. How can I build a lambda expression that will behave something like :
if (dbTable.SingleOrDefault(m => m.incommingtableName_id == MyId))
DoExists();
Any idea ?
You can build an expression in runtime. And also you would need to have generic version of SingleOrDefault method. Here is example:
Type tableType = typeof (incommingtableName); // table type
string idPropertyName = "ID"; // id property name
int myId = 42; // value for searching
// here we are building lambda expression dynamically. It will be like m => m.ID = 42;
ParameterExpression param = Expression.Parameter(tableType, "m");
MemberExpression idProperty = Expression.PropertyOrField(param, idPropertyName);
ConstantExpression constValue = Expression.Constant(myId);
BinaryExpression body = Expression.Equal(idProperty, constValue);
var lambda = Expression.Lambda(body, param);
// then we would need to get generic method. As SingleOrDefault is generic method, we are searching for it,
// and then construct it based on tableType parameter
// in my example i've used CodeFirst context, but it shouldn't matter
SupplyDepot.DAL.SupplyDepotContext context = new SupplyDepotContext();
var dbTable = context.Set(tableType);
// here we are getting SingleOrDefault<T>(Expression) method and making it as SingleOrDefault<tableType>(Expression)
var genericSingleOrDefaultMethod =
typeof (Queryable).GetMethods().First(m => m.Name == "SingleOrDefault" && m.GetParameters().Length == 2);
var specificSingleOrDefault = genericSingleOrDefaultMethod.MakeGenericMethod(tableType);
// and finally we are exexuting it with constructed lambda
var result = specificSingleOrDefault.Invoke(null, new object[] { dbTable, lambda });
As possible optimization constructed lambda can be cached, so we wont need to build it each time, but it should work the same