Expression Expressions

Question

So, I'm trying to figure out expression trees. I am trying to add the dynamic equivalent of Queryable, where T is one of several different tables. First, I check that the table contains a field in which I want to include a filter.

ParameterExpression param = Expression.Parameter(typeof(TSource), "x");

Expression conversionExpression = Expression.Convert(Expression.Property(param, _sourceProperty), typeof(TList));

Expression<Func<TSource, TList>> propertyExpression = Expression.Lambda<Func<TSource, TList>>(conversionExpression, param);

Expression<Func<TList, TList, bool>> methodExpression = (x, y) => x.Equals(y);

ReadOnlyCollection<ParameterExpression> parameters = propertyExpression.Parameters;

InvocationExpression getFieldPropertyExpression = Expression.Invoke(
                                                propertyExpression,
                                                parameters.Cast<Expression>());

MethodCallExpression methodBody = methodExpression.Body as MethodCallExpression;
MethodCallExpression methodCall = Expression.Call(methodBody.Method, Expression.Constant(equalTo), getFieldPropertyExpression);

Expression<Func<TSource, bool>> equalsStatement = Expression.Lambda<Func<TSource, bool>>(methodCall, parameters);

return source.Where(equalsStatement);

When I do this, I have a problem with MethodInfo in the Call statement. This tells me:

The static method requires a null instance, the non-static method requires a non-empty instance.

I am not the owner of expression trees, but I think I understand about 75% of what I am doing here and I know what I'm trying to achieve. TList is a bad name right now, but I took it from an example that works to make the In statement just great.

I'm really looking for an explanation here, so I can work with the code myself, or with an explanation of what I was missing.

Edit:

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ParameterExpression sourceObject = Expression.Parameter(typeof(TSource), "x");

Expression<Func<TSource, bool>> check = Expression.Lambda<Func<TSource, bool>>
        (
            Expression.Equal(
            Expression.MakeMemberAccess(sourceObject, typeof(TSource).GetProperty(_sourceProperty)),
            Expression.Constant(equalTo)
        ),
        sourceObject
);

return source.Where(check);

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Answer 1

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