添加没有值的参数

我有这个存储过程：

CREATE PROCEDURE SearchEmployee
@EmployeeID int,
@EmployeeName nvarchar(256),
@Address nvarchar(256),
AS
Select EmployeeId, EmployeeName, Address 
From Employee
Where
EmployeeID = @EmployeeID OR
EmployeeName LIKE '%' + @EmployeeName+ '%' OR
Address LIKE '%' + @Address+ '%' 

然后在我的Winforms中，我称之为comboBox1.Text=ALL

using (SqlCommand mySqlCommand1 = new SqlCommand("dbo.SearchEmployee", myDatabaseConnection))
  {
  mySqlCommand1.CommandType = CommandType.StoredProcedure;
  if (comboBox1.Text == "ALL")
        {
         mySqlCommand1.Parameters.AddWithValue("@EmployeeID", textBox1.Text);
         mySqlCommand1.Parameters.AddWithValue("@EmployeeName", textBox1.Text);
         mySqlCommand1.Parameters.AddWithValue("@Address", textBox1.Text);
        }
  }

现在，当comboBox1.Text=EmployeeID 时，我将如何操作

像这样的事情：？

      if (comboBox1.Text == "ALL")
        {
         mySqlCommand1.Parameters.AddWithValue("@EmployeeID", textBox1.Text);
         mySqlCommand1.Parameters.AddWithValue("@EmployeeName", novalue);
         mySqlCommand1.Parameters.AddWithValue("@Address", novalue);
        }

if (comboBox1.Text == "EmploueeID")
{
   mySqlCommand1.Parameters.AddWithValue("@EmployeeID", textBox1.Text);
   mySqlCommand1.Parameters.AddWithValue("@EmployeeName",DbNull.Value);
   mySqlCommand1.Parameters.AddWithValue("@Address", DbNull.Value);
}

mySqlCommand1.Parameters.AddWithValue("@EmployeeName", System.DBNull.Value);