Question

Why do I get the following to errors when I try to add ref to an overloaded method’s parameter?

The best overloaded method match for
‘WindowsFormsApplication1.Form1.SearchProducts(int)’ has some invalid
arguments

Argument 1: cannot convert from ‘ref
System.Collections.Generic.List’ to ‘int’

Here’s some (simplified) code:

public virtual IList<int> SearchProducts(int categoryId)
{
    List<int> categoryIds = new List<int>();
    if (categoryId > 0)
        categoryIds.Add(categoryId);
    return SearchProducts(ref categoryIds);
}

public virtual IList<int> SearchProducts(ref IList<int> categoryIds)
{
    return new List<int>();
}

Edit:

Some of you asked me why I need ref in this case and the answer is that I probably don’t need it, because I can clear the list and add new elements (I don’t need to create a new reference). But the question is not about the fact that I need or don’t need ref, it’s about why I got the errors. And since I didn’t find an answer (after googling for little while) I thought the question was interesting and worth asking here. It seems that some of you don’t think it’s a good question and voted to close it down …

Answer 1

When you pass an argument by reference, the compile-time type has to be the exact same type as the parameter type.

Suppose the second method was written as:

public virtual IList<int> SearchProducts(ref IList<int> categoryIds)
{
    categoryIds = new int[10];
    return null;
}

That must compile, as int[] implements IList<int>. However, it would break type safety if the caller actually had a variable of type List<int>, which now had a reference to an int[]…

You can fix this by making the declared type of categoryIds in the calling method IList<int> instead of List<int> – but I strongly suspect you don’t actually want to pass the argument by reference in the first place. It’s relatively rare to need to do so. How comfortable are you with C# parameter passing?