C# nullable string error

System.String is a reference type and already "nullable".

Nullable<T> and the ? suffix are for value types such as Int32, Double, DateTime, etc.

You are making it complicated. string is already nullable. You don't need to make it more nullable. Take out the ? on the property type.

string cannot be the parameter to Nullable because string is not a value type. String is a reference type.

string s = null; 

is a very valid statement and there is not need to make it nullable.

private string typeOfContract
    {
      get { return ViewState["typeOfContract"] as string; }
      set { ViewState["typeOfContract"] = value; }
    }

should work because of the as keyword.