Lambda type parameter must be derived from System.Delegate

Question

In a C# project, I want to create an extension that would take a list of property names and create a dynamic select query.

I found magic Gist that seems to be doing exactly that. However, var lambda = ... keeps throwing the following error

Lambda type parameter must be derived from System.Delegate

Here is the code

public static IQueryable<dynamic> ToDynamic<T>(this IQueryable<T> query, IEnumerable<String> fields)
{
    var pocoType = typeof(T);

    var itemParam = Expression.Parameter(pocoType, "x");
    var members = fields.Select(f => Expression.PropertyOrField(itemParam, f));
    var addMethod = typeof(IDictionary<string, object>).GetMethod(
                "Add", new Type[] { typeof(string), typeof(object) });


    var elementInits = members.Select(m => Expression.ElementInit(addMethod, Expression.Constant(m.Member.Name), Expression.Convert(m, typeof(object))));

    var expando = Expression.New(typeof(ExpandoObject));
    var lambda = Expression.Lambda<Expression<Func<T, dynamic>>>(Expression.ListInit(expando, elementInits), itemParam);

    return query.Select(lambda.Compile());
}

How can I correct this error?

This may be relevant: stackoverflow.com/questions/25793736/… — Evan Trimboli
– Evan Trimboli, Commented Mar 5, 2019 at 0:34
Google it. It is very simple: iamabhik.wordpress.com/tag/… — Al Kepp
– Al Kepp, Commented Mar 5, 2019 at 0:36

Oleksandr · Accepted Answer · 2019-03-05 00:55:20Z

1

In Expression.Lambda<T> expression T is considered lambda type parameter that must be derived from System.Delegate. So you just need to remove wrapping Expression and also there is no need to call Compile at the end since IQueryable expect to receive Expression<Func<T, F>> and not a Func<T, F> which is for IEnumerable

//..
var lambda = Expression.Lambda<Func<T, dynamic>>(Expression.ListInit(expando, elementInits), itemParam);

return query.Select(lambda);

answered Mar 5, 2019 at 0:55

Oleksandr

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4 Comments

Junior Over a year ago

Thank you! Although the var lambda = ... is no longer throwing the previous exception, I am getting the following error ` Only list initializer items with a single element are supported in LINQ to Entities.` when calling the method users.ToDynamic(fields).ToList();

Oleksandr Over a year ago

@MikeA A quick fix would be change method to return IEnumerable<dynamic> and change return statement to return query.AsEnumerable().Select(lambda.Compile()); but it means that query won't be executed in database but on client

Junior Over a year ago

The whole purpose of doing what I am trying to do is to do the select on the database, not in memory. Is there a way do this on the database server?

Oleksandr Over a year ago

@MikeA Well, it's not possible with Dictionary or with ExpandoObject but I can try to investigate a solution with list contains key-value pairs.

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