Question

I have this C# WinForms code in which I have several different structs, that all function in the same way. So instead of writing individual functions for adding or removing items, I’m trying to use Templates instead.

For example, here is one struct and the corresponding List<> I’m using to store its objects:

public struct Alias
{
    public string alias;
    public string aliasSource;

    public static bool IsValid(...); //This function exists in all the structs
};

List<Alias> aliases;

This the function used from the outside, to add Aliases:

public void AddAlias(Alias iAlias)
{
    AddGenericStructItem<Alias>(iAlias, aliases);
}

And this is the actual function doing the addition:

private void AddGenericStructItem<T>(T genericStructItem, List<T> genericList)
{
    string outputError;
    if (T.IsValid(genericStructItem, out outputError)) //< -- Problem in the 'T' being used in the far left
    {
        if (genericList.Contains(genericStructItem))
        {
            MessageBox.Show("ERROR 82ha5jb :: Item already exists");
        }
        else
        {
            genericList.Add(genericStructItem);
        }
    }
    else
    {
        MessageBox.Show(outputError);
    }
}

The problem occurs in the T.IsValid... part. The compiler gives me the following error on the T:

'T' is a 'type parameter', which is not valid in the given context

Is there any way around this? All my structs have an IsValid function in them with the same setup, so it would seem silly to repeatedly write the same code, in case I don’t use templates here…

Answer 1

You can’t use constraints to tell the compiler that a static method will exist on your object. If it really needs to be static, you’ll need to use reflection to call the method:

var methodInfo = typeof(T).GetMethod("IsValid", BindingFlags.Static|BindingFlags.Public);
if (methodInfo != null)
{
    object[] parameters = new object[] { genericStructItem, null };
    if ((bool)methodInfo.Invoke(null, parameters))
    {
        // It's valid!
    }
    else
    {
        string error = (string)parameters[1];
    }
}