试图从的ExecuteScalar获得价值时，铸造例外（） [英] Casting Exception when trying to get value from ExecuteScalar()

问题描述

在下面的代码中，语句1抛出铸造例外。我很奇怪，为什么不是它拆箱？结果
语句2工作正常，但我想知道为什么第一个是错的？

 使用（IDbCommand的命令= connection.CreateCommand（））
 {
 command.CommandText =的String.Format（SELECT COUNT（1）FROM {0}中，TableName）; 
诠释计数=（int）的command.ExecuteScalar（）; //声明1 
} 
 //诠释计数= Convert.ToInt32（command.ExecuteScalar（））; //声明2 
  

解决方案

叹息，执行标量返回长/ Int64的，看到你写的SQL命令你知道回报淡水河谷将是一个计算的整数（SELECT COUNT（...，也可以选择MAX（...

忽略其他方面的建议，你需要的是一个演员。（NO这些命令将永远不会返回一个字符串，如42）

 诠释计数=（INT）（长）command.ExecuteScalar（）; 
  

或担心大量

 长数=（长）command.ExecuteScalar（）; 
  

除非，出于某种奇怪的原因你没有在SQL语句控制执行的问题为什么用转换或装箱/拆箱$ b复杂$ b好格雷夫人，KISS，就用代码膨胀，......，只是回答问题。

In the below code, the statement 1 throws casting exception. I am wondering why isnt it unboxing?
The statement 2 works fine but I want to know why the first one is wrong?

using (IDbCommand command = connection.CreateCommand())
{
    command.CommandText = string.Format("SELECT COUNT(1) FROM {0}", tableName);
    int count = (int)command.ExecuteScalar(); //statement 1
}
//int count = Convert.ToInt32(command.ExecuteScalar()); //statement 2

解决方案

Sigh, execute scalar returns a long/int64, seeing as you wrote the SQL command you know the return vale is going to be a computed whole number (SELECT COUNT(..., also SELECT MAX(...

Ignore the other advice, all you need is a cast. (NO these commands will never return a string, i.e. "42")

int count = (int)(long)command.ExecuteScalar();

or if worried about large numbers

long count = (long)command.ExecuteScalar();

Unless, for some weird reason you don't have control over the SQL statement being executed why complicate matters with Convert, or boxing/unboxing. Good greif people, K.I.S.S., down with code bloat, ... and just answer the question.

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