無法將類型爲「System.DBNull」的對象轉換爲鍵入「System.String」。

Question

我手邊有一個問題，我似乎無法弄清楚。我有這個Button可以觸發一個OnClick。在這個事件中，我正在調用SQL服務器中的存儲過程。在創建新記錄之前，我有一個檢查是否已經存在。如果這樣做，那麼我輸出一個錯誤。這部分工作正常，但是當記錄不存在時，我在ASP.NET中出現此錯誤：

「無法將類型爲'System.DBNull'的對象轉換爲鍵入'System.String'。」

任何幫助將非常感激

protected void createloctype_Click(object sender, EventArgs e) 
{ 
    con.Open(); 
    cmd = new SqlCommand("sp_CREATE_LOCATION_TYPE", con); 
    cmd.CommandType = CommandType.StoredProcedure; 
    cmd.Parameters.Add("@lt_code", SqlDbType.VarChar, 2).Value = loctype.Text; 
    cmd.Parameters.Add("@lt_description", SqlDbType.VarChar, 50).Value = loctypedescription.Text; 
    cmd.Parameters.Add("@lt_putaway_zone", SqlDbType.VarChar, 5).Value = putaway_zone_txt.Text; 
    cmd.Parameters.Add("@lt_rule1", SqlDbType.VarChar, 5).Value = rule1_txt.Text; 
    cmd.Parameters.Add("@lt_rule2", SqlDbType.VarChar, 5).Value = rule2_txt.Text; 
    cmd.Parameters.Add("@lt_rule3", SqlDbType.VarChar, 5).Value = rule3_txt.Text; 
    cmd.Parameters.Add("@lt_rule4", SqlDbType.VarChar, 5).Value = rule4_txt.Text; 
    cmd.Parameters.Add("@lt_rule5", SqlDbType.VarChar, 5).Value = rule5_txt.Text; 
    cmd.Parameters.Add("@lt_misc1", SqlDbType.VarChar, 500).Value = misc1_txt.Text; 
    cmd.Parameters.Add("@lt_misc2", SqlDbType.VarChar, 500).Value = misc2_txt.Text; 
    cmd.Parameters.Add("@lt_misc3", SqlDbType.VarChar, 500).Value = misc3_txt.Text; 
    cmd.Parameters.Add("@lt_misc4", SqlDbType.VarChar, 500).Value = misc4_txt.Text; 
    cmd.Parameters.Add("@lt_misc5", SqlDbType.VarChar, 500).Value = misc5_txt.Text; 
    cmd.Parameters.Add("@lt_user", SqlDbType.VarChar, 10).Value = user; 
    cmd.Parameters.Add("@error", SqlDbType.VarChar, 250); 
    cmd.Parameters["@error"].Direction = ParameterDirection.Output; 
    cmd.ExecuteNonQuery();     
    message = (string)cmd.Parameters["@error"].Value; 

    error_message.Text = message; 

    con.Close(); 

} 

/存儲過程/

USE [1_WMS] 

GO 

ALTER PROCEDURE [dbo].sp_CREATE_LOCATION_TYPE 
@LT_CODE VARCHAR(5) 
, @LT_DESCRIPTION VARCHAR(250) 
, @LT_PUTAWAY_ZONE VARCHAR(5) 
, @LT_RULE1 VARCHAR(5) 
, @LT_RULE2 VARCHAR(5) 
, @LT_RULE3 VARCHAR(5) 
, @LT_RULE4 VARCHAR(5) 
, @LT_RULE5 VARCHAR(5) 
, @LT_MISC1 VARCHAR(20) 
, @LT_MISC2 VARCHAR(20) 
, @LT_MISC3 VARCHAR(20) 
, @LT_MISC4 VARCHAR(20) 
, @LT_MISC5 VARCHAR(20) 
, @LT_USER VARCHAR(20) 
, @ERROR VARCHAR(250) OUT 

AS 
DECLARE 
    @LT_DATE VARCHAR(10) 
    , @LT_TIME VARCHAR(8) 
    , @LT_ROW_COUNT INT 

SET @LT_DATE = CONVERT(VARCHAR(10), GETDATE(),101) 
SET @LT_TIME = CONVERT(VARCHAR(8), GETDATE(),114) 

/* CHECK FOR EXISTING */ 
SELECT @LT_ROW_COUNT = COUNT(*) 
    FROM LOC_TYPE(NOLOCK) 
WHERE lt_code = @LT_CODE 

/* */ 
IF @LT_ROW_COUNT = 0 
    BEGIN 
     INSERT INTO LOC_TYPE 
      (
       lt_code    , lt_description   , lt_putaway_zone    , lt_rule_1 
       , lt_rule_2   , lt_rule_3     , lt_rule_4      , lt_rule_5 
       , lt_misc1   , lt_misc2     , lt_misc3      , lt_misc4 
       , lt_misc5   , lt_created_date   , lt_created_time    , lt_created_by 
       , lt_modify_date , lt_modify_time   , lt_modify_by 
      ) 
      VALUES 
      (
       @LT_CODE   , @LT_DESCRIPTION   , @LT_PUTAWAY_ZONE    , @LT_RULE1 
       , @LT_RULE2   , @LT_RULE3     , @LT_RULE4      , @LT_RULE5 
       , @LT_MISC1   , @LT_MISC2     , @LT_MISC3      , @LT_MISC4 
       , @LT_MISC5   , @LT_DATE     , @LT_TIME      , @LT_USER 
       , @LT_DATE   , @LT_TIME     , @LT_USER 
      ) 
    END 

ELSE IF @LT_ROW_COUNT = 1 
    BEGIN 

     SET @ERROR = 'LOCATIOIN TYPE ALREADY EXIST' 

     EXEC sp_CREATE_ERROR_MESSAGE 
      'CRT_LTY'  , @ERROR   , @LT_CODE   , @LT_PUTAWAY_ZONE 
      , @LT_RULE1  , @LT_RULE2   , @LT_RULE2   , @LT_RULE3 
      , @LT_RULE4  , @LT_RULE5   , ''    , ''   
      , @LT_USER 
    END 


GO 

Answer 1

嘗試的IF

var result = cmd.Parameters["@error"].Value; 
message = (result == DBNull.Value) ? string.Empty : result.ToString(); 

或者乾脆

短版

或

var result = cmd.Parameters["@error"].Value; 
message = ((result == DBNull.Value) || (result == DBNull.Value)) ? string.Empty : result.ToString(); 

Answer 2

較短的替代：

message = cmd.Parameters["@error"].Value as string ?? "";