fix(datatable): handle Nullable<T> and existing columns in AddColumn<T>

Author: engineering87

SharpHelpers/SharpHelpers/DataTableHelper.cs

@@ -78,7 +78,7 @@ string Escape(string s)
             foreach (DataRow row in table.Rows)
                 lines.Add(string.Join(delimiter, row.ItemArray.Select(v => Escape(v?.ToString()))));
             return string.Join(Environment.NewLine, lines);
-        }
+        }
 
         /// <summary>
         /// Adds a new column to the DataTable with the specified default value.
@@ -87,17 +87,24 @@ string Escape(string s)
         /// <param name="columnName"></param>
         /// <param name="defaultValue"></param>
         /// <typeparam name="T"></typeparam>
-        public static void AddColumn<T>(this DataTable table, string columnName, T defaultValue = default)
+        public static void AddColumn<T>(this DataTable table, string columnName, T defaultValue = default!)
         {
             if (table == null) throw new ArgumentNullException(nameof(table));
-
-            var column = new DataColumn(columnName, typeof(T)) { DefaultValue = defaultValue };
-            table.Columns.Add(column);
+            if (string.IsNullOrWhiteSpace(columnName)) throw new ArgumentException("Empty", nameof(columnName));
+            if (table.Columns.Contains(columnName)) 
+                return;
+
+            var t = Nullable.GetUnderlyingType(typeof(T)) ?? typeof(T);
+            var col = new DataColumn(columnName, t) 
+            { 
+                DefaultValue = defaultValue! 
+            };
+            table.Columns.Add(col);
             foreach (DataRow row in table.Rows)
             {
-                row[columnName] = defaultValue;
+                row[columnName] = defaultValue!;
             }
-        }
+        }
 
         /// <summary>
         /// Merges multiple DataTables with the same schema into one.