C# 通过this关键字来扩展方法

public static class ExtString
    {
        public static string myTest(this String str)
        {
            return "[" + str + "]";
        }
        public static DataRow Expect(this DataRow dr, string fieldName)
        {
            if (dr != null)
            {
                DataTable newDt = dr.Table;
                newDt.Columns.Remove(fieldName);
                DataRow newDr = newDt.NewRow();
                foreach (DataColumn dc in dr.Table.Columns)
                {
                    if (!dc.ColumnName.Equals(fieldName))
                    {
                        newDr[dc.ColumnName] = dr[dc.ColumnName];
                    }
                }

                return newDr;
            }
            return null;

        }

    }

string m1 = "ddd";
            MessageBox.Show(m1.myTest());
            
            
            DataTable dt = new DataTable();
            dt.Columns.Add("TID", typeof(int));
            dt.Columns.Add("Flg", typeof(int));
            dt.Columns.Add("TName", typeof(string));
            DataRow dr = dt.NewRow();
            dr["TID"]=1;
            dr["Flg"] = 1;
            dr["TName"] = "testaa";
            //dt.Rows.Add(dr);

            DataRow dr1 = dr.Expect("Flg");

public static class DataContractExtensions
{
  
//测试方法
  public static T Test<T>(this T instance) where T : Test2
  {
       T Res = default(T);
       try
       {
           Res.AttrTest = instance.AttrTest.Substring(0,2);
           
//其他复杂逻辑...
      }
      catch
      { }
      return Res;
  }
}
public class Test2
{
　　public string AttrTest { set; get; }
}