C# Optional Arguments

How do you implement optional arguments or parameters in c#?
An easy way to do this is:

public boolean isValidFName(string FName, int MinLength = 0)
{
    if (FName.Length == 0 || FName.Length < MinLength)
       return false;
    else
       return true;
}



if (isValidFName(txtFName.Text))
{
    //Only checks that the first name is not length 0.
}


if (isValidFName(txtFName.Text, 5))
{
    //Checks that the first name is at least length of 5.
}