不允许从数据类型nvarchar到varbinary(max)的隐式转换

Question

当我尝试将DBNull.Value插入可空的 varbinary(max)字段时,我得到此异常:

Implicit conversion from data type nvarchar to varbinary(max) is not allowed. Use the CONVERT function to run this query.

那是我的代码:

  insertCMD.Parameters.AddWithValue("@ErrorScreenshot", SqlDbType.VarBinary).Value = DBNull.Value;

我知道在SO上存在重复的问题,但我不像其他人那样使用任何字符串.

我错了什么？

更新:

using (var insertCMD = new SqlCommand("INSERT INTO TestplanTeststep (TeststepId,TestplanId,CreatedAt,ErrorText,ErrorScreenshot,TestState) VALUES (@TeststepId, @TestplanId,@CreatedAt,@ErrorText,@ErrorScreenshot,@TestState)", con))
{
 var p1 = insertCMD.Parameters.Add("@TeststepId", SqlDbType.Int);
 var p2 = insertCMD.Parameters.Add("@CreatedAt", SqlDbType.DateTime);
 insertCMD.Parameters.AddWithValue("@TestplanId", testplan.Id);
 insertCMD.Parameters.AddWithValue("@ErrorText", (object) DBNull.Value);
 insertCMD.Parameters.AddWithValue("@ErrorScreenshot", (object) DBNull.Value);
 insertCMD.Parameters.AddWithValue("@TestState", (int)Teststep.TeststepTestState.Untested);

       foreach (Teststep step in teststeps)
        {
           p1.Value = step.Id;
           p2.Value = step.CreatedAt;
           insertCMD.ExecuteNonQuery();
        }
     }

Answer 1

我有同样的问题,而插入DBNull.Value的Varbinary(Max)列.谷歌搜索后,我找到了一个可能对你有帮助的解决方案:

在添加sql参数时,需要设置大小-1,这意味着Maxvarbinary列的长度:

this.cmd.Parameters.Add("@Photo", SqlDbType.VarBinary, -1).Value = DBNull.Value;

所以在你的情况下:

insertCMD.Parameters.Add("@ErrorScreenshot", SqlDbType.VarBinary,-1).Value = DBNull.Value;

Answer 2

为什么不将SQL更改为:

INSERT INTO TestplanTeststep
(TeststepId,TestplanId,CreatedAt,ErrorText,ErrorScreenshot,TestState) 
VALUES 
(@TeststepId, @TestplanId,@CreatedAt,NULL,NULL,@TestState)

要不就

INSERT INTO TestplanTeststep
(TeststepId,TestplanId,CreatedAt,TestState) 
VALUES 
(@TeststepId, @TestplanId,@CreatedAt,@TestState)

...并省略这两个参数？

如果它总是NULL,那将具有相同的效果.

否则,请尝试两行:

var binary1 = insertCMD.Parameters.Add("@ErrorScreenshot", SqlDbType.VarBinary, -1);
binary1.Value = DBNull.Value;

否则,在原始SQL插入语句中,您没有定义参数类型,而是传入varbinary,因此错误.