How to insert update multiple records in sql table?

alter PROCEDURE Sp_AssignProjectToUser
(

 @Ref_UserID int =  null
, @Ref_ProjectID int  = null
, @ActiveFlag bit  =null
)
AS
  BEGIN
    INSERT INTO UserMappingProject (Ref_UserID
    , Ref_ProjectID
    , ActiveFlag)
      VALUES (@Ref_UserID, @Ref_ProjectID, @ActiveFlag)
  END

public int AssignProjectToUser(DataTable dtUserProjectMap, Int32 batchSize)
       {
int Result = 0;
            SqlConnection con = new SqlConnection(connectionString);
            SqlCommand cmd = new SqlCommand("Sp_AssignProjectToUser", con);

            cmd.Parameters.AddWithValue("@Ref_UserID", Ref_UserID);
            cmd.Parameters.AddWithValue("@Ref_ProjectID", ProjectID);
            cmd.Parameters.AddWithValue("@ActiveFlag ", flag);
            con.Open();
            cmd.CommandType = CommandType.StoredProcedure;
            Result = cmd.ExecuteNonQuery();
            con.Close();
            return Result;
            SqlDataAdapter adapter = new SqlDataAdapter();

            // Set the UPDATE command and parameters.
            adapter.UpdateCommand = new SqlCommand("Sp_AssignProjectToUser", con);
            adapter.UpdateCommand.Parameters.Add("@Ref_UserID", SqlDbType.Int, 5, "Ref_UserID");
            adapter.UpdateCommand.Parameters.Add("@Ref_ProjectID", SqlDbType.Int, 4, "Ref_ProjectID");
            adapter.UpdateCommand.Parameters.Add("@ActiveFlag", SqlDbType.Bit, 1, "ActiveFlag");
            adapter.UpdateCommand.UpdatedRowSource = UpdateRowSource.None;
            adapter.UpdateBatchSize = 4;


             Set the INSERT command and parameter.
            adapter.InsertCommand = new SqlCommand("Sp_AssignProjectToUser", con);
            adapter.InsertCommand.Parameters.AddWithValue("@Ref_UserID","Ref_UserID");
            adapter.InsertCommand.Parameters.Add("@Ref_UserID", SqlDbType.Int, 5, "Ref_UserID");
            adapter.InsertCommand.Parameters.Add("@Ref_ProjectID", SqlDbType.Int, 4, "Ref_ProjectID");
            adapter.InsertCommand.Parameters.Add("@ActiveFlag", SqlDbType.Bit, 1, "ActiveFlag");

            adapter.InsertCommand.UpdatedRowSource = UpdateRowSource.None;

     
            // Set the batch size.
            adapter.UpdateBatchSize = batchSize;

           adapter.Update(dtUserProjectMap);
            return Result;

Cannot insert the value NULL into column 'Ref_UserID', table 'ZP505_VersionControlTracker.dbo.UserMappingProject'; column does not allow nulls. INSERT fails.
Cannot insert the value NULL into column 'Ref_UserID', table 'ZP505_VersionControlTracker.dbo.UserMappingProject'; column does not allow nulls. INSERT fails.