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

Answer 1

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
);