Question

Using C# 2.0, I can specify a default parameter value like so:

static void Test([DefaultParameterValueAttribute(null)] String x) {}

Since this C# 4.0 syntax isn’t available:

static void Test(String x = null) {}

So, is there a C# 2.0 equivalent for value types? For instance:

static void Test(int? x = null) {}

The following attempts don’t compile.

// error CS1908: The type of the argument to the DefaultValue attribute must match the parameter type
static void Test([DefaultParameterValueAttribute(null)] int? x) {}

// error CS0182: An attribute argument must be a constant expression, typeof expression or array creation expression
static void Test([DefaultParameterValueAttribute(new Nullable<int>())] int? x) {}

Answer 1

Unfortunately, the older versions of the C# compiler do not support this.

The C# 4.0 compiler compiles this:

public static void Foo(int? value = null)

Into:

public static void Foo([Optional, DefaultParameterValue(null)] int? value)

This is effectively the same as your first attempt (with OptionalAttribute added, in addition), which the C# 2 compiler errors on with CS1908, as this wasn’t supported directly in that version of the compiler.

If you need to support C# 2, in this case, I would recommend adding an overloaded method instead:

static void Test()
{
    Test(null);
}
static void Test(int? x)
{
    // ..