Parameterattribute default value (C# cmdlet) in PowerShell

With auto-implemented properties, you can't. You will need to create the actual getter and setter.

Something like this:

private string _ParameterName = "defaultvalue";

[Parameter]
public string ParameterName 
{
     get
     {
          return _ParameterName ;
     }
     set
     {
         _ParameterName  = value;
     }
}

Since C# 6.0 has been released:

[Parameter]
public string ParameterName { get; set; } = "defaultValue";