Column Mapping Using Expressions and Dictionary

Question

I'm having trouble coding an expression for IQueryable object. I can't figure out the right way to use the expression to map a string to object properties.

this is my query object and mapping dictionary:

var query = context.Industries.AsQueryable();
var columnsMap = new Dictionary<string, Expression<Func<Industry, object>>>()
            {
                ["name"] = v => v.Name,
                ["isicCode"] = v => v.IsicCode.Data,
                ["isicCodeTitle"] = v => v.IsicCode.Title,
                ["isActive"] = v => v.IsActive,
            };

and I'm using columnsMap dictionary for applying Orderby to my query in an extentions class:

public static IQueryable<T> ApplyOrdering<T>(this IQueryable<T> query, IQueryObject queryObj, Dictionary<string, Expression<Func<T, object>>> columnsMap)
{
    if (String.IsNullOrWhiteSpace(queryObj.SortBy) || !columnsMap.ContainsKey(queryObj.SortBy))
        return query;
    if (queryObj.IsSortAsc)
        return query.OrderBy(columnsMap[queryObj.SortBy]);
    else
        return query.OrderByDescending(columnsMap[queryObj.SortBy]);
}

it's ok for OrderBy but I need to do the same thing for ApplyFiltering but for filter an IQueryable object I need a different expression for where method Expression<Func<T, bool>>

public static IQueryable<T> ApplyFiltering<T>(this IQueryable<T> query, IQueryObject queryObj, Dictionary<string, Expression<Func<T, object>>> columnsMap)
{           
    query.Where(columnsMap['name'] == "test Name"); //this is the problem. 
    return query;
}

the question is how can I use my columnsMap in ApplyFiltering method? or should I change my columnsMap for that?

Answer 1

It is quite simple. The only problem is when you have a value type column (an int or a bool or a DateTime for example)... An Expression<Func<Industry, object>> will introduce a boxing of the field/property to object that we have to remove. This problem is absent for strings.

// isActive is a bool
Expression<Func<Industry, object>> exp = columnsMap["isActive"];
object value = true; // You can't use "true" (string) here! isActive is a bool

// Other exammple
// Expression<Func<Industry, object>> exp = columnsMap["name"];
// object value = "Foo";

var body = exp.Body;

// Remove the boxing for value types
if (body.NodeType == ExpressionType.Convert)
{
    body = ((UnaryExpression)body).Operand;
}

var eq = Expression.Equal(body, Expression.Constant(value, body.Type));

var exp2 = Expression.Lambda<Func<T, bool>>(eq, exp.Parameters);

return query.Where(exp2);

Example using string value:

Expression<Func<Industry, object>> exp = columnsMap["isActive"];
string value = "true";
// Other exammple
//Expression<Func<Industry, object>> exp = columnsMap["name"];
//string value = "Foo";

var body = exp.Body;

// Remove the boxing for value types
if (body.NodeType == ExpressionType.Convert)
{
    body = ((UnaryExpression)body).Operand;
}

object value2 = value;

if (value2 != null && body.Type != value2.GetType())
{
    value2 = Convert.ChangeType(value2, body.Type);
}

var eq = Expression.Equal(body, Expression.Constant(value2, body.Type));

var exp2 = Expression.Lambda<Func<Industry, bool>>(eq, exp.Parameters);

return query.Where(exp2);