Size属性的大小0无效

这是我的SP:

CREATE PROCEDURE _insertAdminProfile
(
    @Ad_Fname VARCHAR(25),
    @Ad_Lname VARCHAR(25),
    @Ad_DOB DATE,
    @Ad_Sex VARCHAR(25),
    @Ad_Address VARCHAR(200),
    @Ad_Phone VARCHAR(12),
    @Ad_Email VARCHAR(25),
    @Ad_Pwd VARCHAR(7) OUTPUT
)
AS BEGIN
    DECLARE @MyTableVar TABLE (Ad_Pwd VARCHAR(7));
    INSERT INTO dbo.Admin_Profile (Ad_Fname, Ad_Lname, Ad_DOB, Ad_Sex,Ad_Address,Ad_Phone,Ad_Email)
    OUTPUT INSERTED.Ad_Regid INTO @MyTableVar
    VALUES (@Ad_Fname, @Ad_Lname, @Ad_DOB, @Ad_Sex, @Ad_Address, @Ad_Phone, @Ad_Email)
    SELECT @Ad_Pwd = Ad_Pwd
    FROM @MyTableVar
END

这是我的代码：

protected void cmdprofile_Click(object sender, EventArgs e)
{
    string _gender;
    if (rdomale.Checked)
    {
       _gender = "Male";
    }
    else
    {
       _gender = "Female";
    }
    Class1.con = CreateConnection();
    cmd = new SqlCommand("_insertAdminProfile", Class1.con );
    cmd.CommandType =  System.Data.CommandType.StoredProcedure;
    cmd.Parameters.AddWithValue("@Ad_Fname", txtfname.Text);
    cmd.Parameters.AddWithValue("@Ad_Lname", txtlname.Text);
    cmd.Parameters.AddWithValue("@Ad_DOB", txtdob.Text);
    cmd.Parameters.AddWithValue("@Ad_Sex", _gender);
    cmd.Parameters.AddWithValue("@Ad_Address", txtaddress.Text);
    cmd.Parameters.AddWithValue("@Ad_Phone", txtphone.Text);
    cmd.Parameters.AddWithValue("@Ad_email", txtemail.Text);
    SqlParameter _outputparam = new SqlParameter();
    _outputparam.ParameterName = "@Ad_Regid";
    _outputparam.SqlDbType = System.Data.SqlDbType.VarChar;
    _outputparam.Direction = System.Data.ParameterDirection.Output;
     cmd.Parameters.Add(_outputparam); 
     cmd.ExecuteNonQuery(); **// Here shows my error:: String[7]: the Size property has an invalid size of 0.**
     string _ID = _outputparam.Value.ToString();
     ClientScript.RegisterStartupScript(this.GetType(), "message", "alert('New Values Inserted and Your temporary password is: " + _ID + "');", true);          
     MultiView1.SetActiveView(Login_stat);
}

   _outputparam.Size = 7;