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c#linqienumerablecontainsexpressionbuilder
Shin
Shin

Reputation: 673

How to build a LINQ expression with Contains method from IEnumerable?

I'm trying to build a LINQ expression, to filter values from a int property:

protected IQueryable<T> Some(IEnumerable<int> ids)
{
    var parameter = Expression.Parameter(typeof(T), "x");
    // "Col_id" (int property)
    var property = Expression.Property(parameter, "Col_id");

    MethodInfo method = typeof(Enumerable).
                        GetMethods().
                        Where(x => x.Name == "Contains").
                        Single(x => x.GetParameters().Length == 2).
                        MakeGenericMethod(typeof(T));

    // ids = { 2, 4, 8 } etc...
    var value = Expression.Constant(ids, typeof(IEnumerable<int>));
    var containsMethod = Expression.Call(method, property, value); // exception

    var aDelegate = Expression.Lambda<Func<T, bool>>(containsMethod, parameter);

    table = myDataContext.GetTable<T>();

    return table.AsQueryable().Where(aDelegate);
}

I'm trying to get something like: (x => ids.Contains(x.Col_id)), but an exception is thrown:

Expression of type 'System.Int32' cannot be used for type parameter 'System.Collections.Generic.IEnumerable'1[T] from 'Boolean Contains[T](System.Collections.Generic.IEnumerable'1[T], T)' method

Upvotes: 2

Views: 5385

Answers (1)

Jon Skeet
Jon Skeet

Reputation: 1500855

It looks to me like you've just got the arguments the wrong way round.

This:

Expression.Call(method, property, value)

means that you're calling:

Enumerable.Contains(x.Col_id, ids)

whereas you want

Enumerable.Contains(ids, x.Col_id)

So just try:

var containsMethod = Expression.Call(method, value, property);

EDIT: Next, you're building the wrong Contains type argument, I think. I suspect you want:

MethodInfo method = typeof(Enumerable).
                    GetMethods().
                    Where(x => x.Name == "Contains").
                    Single(x => x.GetParameters().Length == 2).
                    MakeGenericMethod(typeof(int));

After all, you want to call Enumerable.Contains<int>, not Enumerable.Contains<SomeType>.

Upvotes: 7

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