Trying to build lambda tree

Question

I'm trying to build an expression that must create query of type WHERE IN () For IN I must check if value exist in a list, so my expression looks like:

long loKey = 2;
List<long> loKeys = new List<long>();
loKeys.Add(loKey);

ParameterExpression parameter = Expression.Parameter(type, "t");
var constantExpression = Expression.Constant((List<long>)loKeys, typeof(List<long>));
var lambda = Expression.Lambda(
    typeof(Func<,>).MakeGenericType(type, typeof(bool)),
    Expression.Equal(
        Expression.Property(parameter, "ID"),
        constantExpression
    ),
    parameter
);
resultQuery = resultQuery.Provider.CreateQuery(
    Expression.Call(
        typeof(Queryable), 
        "Where",
        new Type[] { type },
        resultQuery.Expression,
        lambda
    ) 
);

So the idea is that field ID must exist in list loKeys, but I'm getting an error:

Additional information: The binary operator Equal is not defined for the types 'System.Int64' and 'System.Collections.Generic.List`1[System.Int64]'.

On Expression.Lambda -> MakeGenericType

Why don't use something like query.Where(entity => keysEnumerable.Contains(entity.Id)), where keysEnumerable is IEnumerable<PrimaryKeyType>? — Dennis
– Dennis, Commented Mar 28, 2016 at 6:16

Ivan Stoev · Accepted Answer · 2016-03-28 07:36:36Z

3

So you are trying to build something like this:

t => loKeys.Contains(t.ID)

The only thing you need to consider is that actually Contains is a static extension method defined in Enumerable class:

var lambda = Expression.Lambda(
    Expression.Call(
        typeof(Enumerable), 
        "Contains", 
        new[] { typeof(long) },
        Expression.Constant(loKeys),
        Expression.Property(parameter, "ID")
    ),
    parameter
);

answered Mar 28, 2016 at 7:36

Ivan Stoev

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Comments

Vnvizitiu · Accepted Answer · 2016-03-28 06:37:24Z

1

I think it the exception is thrown because of the part: Expression.Equal( Expression.Property(parameter, "ID"), constantExpression ), In which you're comparing the constantExpressions, which is a generic list, with a parameter of type long (i guess, doesn't say what type the parameter is), maybe instead of Equals you should use Method call for "Contains" and see if the parameter is in the list (like Dennis suggested).

edited Mar 28, 2016 at 6:37

answered Mar 28, 2016 at 6:28

Vnvizitiu

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2 Comments

Harry Birimirski Over a year ago

Yes ,yes i thing about the same , but Expression doesn`t have method Contains .. so the real question is what method can be used instead of Equal ?

shay__ Over a year ago

@HarryBirimirski if type is always ICollection, you can use Expression.Call and pass it ICollection.Contains() as methodInfo

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