獲得一個「爲最佳重載的方法匹配......有一些無效參數」的錯誤消息

Question

我有下面的代碼的問題：

public class ClientGroupDetails 
{ 
    public DateTime Col2; 
    public String Col3; 
    public Int32 Col4; 

    public ClientGroupDetails(DateTime m_Col2, String m_Col3, Int32 m_Col4) 
    { 
     Col2 = m_Col2; 
     Col3 = m_Col3; 
     Col4 = m_Col4; 
    } 

    public ClientGroupDetails() { } 
} 

[WebMethod()] 
public List<ClientGroupDetails> GetClientGroupDetails(string phrase) 
{ 
    var client_group_details = new List<ClientGroupDetails>(); 

    using (connection = new SqlConnection(ConfigurationManager.AppSettings["connString"])) 
    { 
     using (command = new SqlCommand(@"select col2, col3, col4 from table1 where col1 = @strSearch", connection)) 
     { 
      command.Parameters.Add("@strSearch", SqlDbType.VarChar, 255).Value = phrase; 

      connection.Open(); 
      using (reader = command.ExecuteReader()) 
      { 
       int Col2Index = reader.GetOrdinal("col2"); 
       int Col3Index = reader.GetOrdinal("col3"); 
       int Col4Index = reader.GetOrdinal("col4"); 

       while (reader.Read()) 
       { 
        client_group_details.Add(new ClientGroupDetails(
         reader.IsDBNull(Col2Index) ? (Nullable<DateTime>)null : (Nullable<DateTime>)reader.GetDateTime(Col2Index), 
         reader.IsDBNull(Col3Index) ? null : reader.GetString(Col3Index), 
         reader.GetInt32(Col4Index))); 
       } 
      } 
     } 
    } 

    return client_group_details; 
} 
} 

這是給我下面的錯誤：

Compiler Error Message: CS1502: The best overloaded method match for 'Conflicts.ClientGroupDetails.ClientGroupDetails(System.DateTime, string, int)' has some invalid arguments 
Line 184: client_group_details.Add(new ClientGroupDetails(

Answer 1

使m_Col2 DateTime?（Nullable<DateTime>）

所以最後它會像這樣

public class ClientGroupDetails 
{ 
    public Nullable<DateTime> Col2; 
    public String Col3; 
    public Int32 Col4; 

    public ClientGroupDetails(Nullable<DateTime> m_Col2, String m_Col3, Int32 m_Col4) 
    { 
     Col2 = m_Col2; 
     Col3 = m_Col3; 
     Col4 = m_Col4; 
    } 

    public ClientGroupDetails() { } 
} 

[WebMethod()] 
public List<ClientGroupDetails> GetClientGroupDetails(string phrase) 
{ 
    var client_group_details = new List<ClientGroupDetails>(); 
    using (connection = new SqlConnection(ConfigurationManager.AppSettings["connString"])) 
    { 
     using (command = new SqlCommand(@"select col2, col3, col4 from table1 where col1 = @strSearch", connection)) 
     { 
      command.Parameters.Add("@strSearch", SqlDbType.VarChar, 255).Value = phrase; 

      connection.Open(); 
      using (reader = command.ExecuteReader()) 
      { 
       int Col2Index = reader.GetOrdinal("col2"); 
       int Col3Index = reader.GetOrdinal("col3"); 
       int Col4Index = reader.GetOrdinal("col4"); 

       while (reader.Read()) 
       { 
        client_group_details.Add(new ClientGroupDetails(
         reader.IsDBNull(Col2Index) ? (Nullable<DateTime>)null : (Nullable<DateTime>)reader.GetDateTime(Col2Index), 
         reader.IsDBNull(Col3Index) ? (string)null : reader.GetString(Col3Index), 
         reader.GetInt32(Col4Index))); 
       } 
      } 
     } 
    } 

    return client_group_details; 
} 
} 

BTW：我會建議你在POCO使用屬性，而不是公共實例變量，並給他們有意義的名字2.使用小寫的局部變量名

Answer 2

的構造函數簽名是DataTime

new ClientGroupDetails(
      reader.IsDBNull(Col2Index) ? (Nullable<DateTime>)null 
     ... 

：和你 Nullable<DataTime>

public ClientGroupDetails(DateTime m_Col2, String m_Col3, Int32 m_Col4) 

稱它爲你有把它稱爲

更改構造函數來：

public ClientGroupDetails(DateTime? m_Col2, string m_Col3, int m_Col4) 

和物業：

public DateTime? Col2; 

注：

• 你不必寫的Int32，int是一個「別名「 爲了它。
• 與Nullable<DateTime> =>DataTime?相同的是它的別名。

Answer 3

爲ClientGroupDetails的構造不採取Nullable<DateTime>（又名DateTime?） - 它需要平DateTime。您需要更改ClientGroupDetails使用的內容，或者考慮默認值（可能是DateTime.MinValue）。

例如::

client_group_details.Add(new ClientGroupDetails(
    reader.IsDBNull(Col2Index) ? DateTime.MinValue : reader.GetDateTime(Col2Index), 
    reader.IsDBNull(Col3Index) ? null : reader.GetString(Col3Index), 
    reader.GetInt32(Col4Index))); 

或者保留現有的閱讀器的代碼，並更改POCO：

public class ClientGroupDetails 
{ 
    public DateTime? Col2; 
    ...  
    public ClientGroupDetails(DateTime? m_Col2, ... 
    { 
     Col2 = m_Col2; 
     ... 
    } 
    ... 
} 

或者 - 使用一個工具，如「短小精悍」，這將做這一切爲你：

var list = connection.Query<ClientGroupDetails>(
     @"select col2, col3, col4 from table1 where col1 = @phrase", 
     new {phrase}).ToList();