Optionally passing a struct to a custom function in C#

There are other similar SO posts on this, but I'm struggling to find anything that makes much sense. Still fairly new to C# here...

So, all I want to do is add an optional parameter to a custom function. This param needs to contain two values so I figured a custom struct would work, like this:

public struct FooBah {
    public string foo;
    public string bah;

    public FooBah(string foo, string bah) {
        this.foo  = foo;
        this.bah = bah;
    }
}

Then to use it in a function call:

FooBah foobah = new FooBah("foo", "bah");
CustomFunction(foobah);

And for the function itself:

private void CustomFunction(FooBah foobah = null) {
    if(foobah != null) {
        // Do something with foobah
    }
}

But this errors because:

> A value of type '<null>' cannot be used as a default parameter because there are no standard conversions to type 'FooBah'

Any advice please?

If there is a default state for the struct that you can detect (such as foo being null) you can write code like this:

static void CustomFunction(FooBah foobah = default)
{
    if (foobah.foo != null)
    {
        // Do something with foobah
    }
}

Of course this depends on your usage of foo.

Alternatively you can use a FooBah? parameter:

static void CustomFunction(FooBah? foobah = null)
{
    if (foobah == null)
    {
        // Do something with foobah
    }
}

The following calls all work:

CustomFunction();
CustomFunction(new FooBah());
CustomFunction(null);

var fooBah = new FooBah();
CustomFunction(fooBah);

Note that if you don't want to modify the struct inside the method you can use the in modifier:

static void CustomFunction(in FooBah? foobah = null)
{
    if (foobah == null)
    {
        Console.WriteLine("Default");
    }
    else
    {
        Console.WriteLine("Not default");
    }
}

Note that when using a nullable value type, you must access the value itself via a .Value property, i.e. foobah.Value.foo - this is because the type is Nullable<FooBah> rather than FooBah.

See here for the Nullable value type documentation

See here for more information on when you should use in.

Finally the most obvious solution is to overload the method with and without a parameter.

This could work, but your structure is not nullable, as in it can't be converted to null, hence, you could do it like this to define an optional argument -

public struct FooBah {
    public string foo;
    public string bah;

    public FooBah(string foo, string bah) {
        this.foo  = foo;
        this.bah = bah;
    }
}

FooBah foobah = new FooBah("foo", "bah");
CustomFunction(foobah);

private void CustomFunction(CustomFunction? structOptional = null){
//What you want to do with the struct
}