var currentPriorityKey = chkbxActionItem.Checked ? cboActionPriority.SelectedValue : "NULL";
 var currentAssigned = chkbxActionItem.Checked ? cboActionAssignedTo.SelectedValue : "NULL";

var addJobNoteSucceeded = db.ExeSQL($"EXEC usp_JobNote @ProjectKey = {ProjectKey}, @EmpGuid = '{EBResources.EmpGuid}', @NoteType = {currentNoteTypeId}, @Title = '{txtTitle.Text}', @Note = '{txtNote.Text}', @ActionItem = {currentActionItem}, @AssignedTo = '{currentAssigned}', @PriorityKey = {currentPriorityKey}, @ChronologicalDate = '{this.dtpDate.Value}', @DesignKey = {cboDesign.SelectedValue}");

public bool ExeSQL(string sql)
        {
            bool bFlag = false;
            SqlCommand cmd = new SqlCommand(sql.ToString(), this.dbconn)
            {
                CommandTimeout = 0,
                CommandType = CommandType.Text
            };
            try
            {
                cmd.ExecuteNonQuery();
                bFlag = true;
            }
            catch (SqlException e)
            {
                this.HandleSQLError(e, "ExeSQL", sql);
            }
            finally
            {
                cmd.Dispose();
            }
            return bFlag;
        }

EXEC usp_JobNote @AssignedTo = 'NULL', @PriorityKey = NULL, @ChronologicalDate = '7/23/2019 4:24:46 PM', @DesignKey = 4031

Error converting data type varchar to uniqueidentifier.