SqlParameter的构造函数编译器过载的选择 [英] SqlParameter constructor compiler overload choice
问题描述
在创建的SqlParameter （.NET3.5）或者 OdbcParameter ，我经常使用的SqlParameter（字符串参数名称，对象的值）构造函数重载设置的值在一个语句。
When creating an SqlParameter (.NET3.5) or OdbcParameter, I often use the SqlParameter(string parameterName, Object value) constructor overload to set the value in one statement.
然而，当我试图通过文字0作为参数的值，我最初是抓住了C＃编译器选择<$的（字符串，OdbcType）过载，而不是C $ C>（字符串对象）。
However, when I tried passing a literal 0 as the value parameter, I was initially caught by the C# compiler choosing the (string, OdbcType) overload instead of (string, Object).
MSDN居然发出警告这一疑难杂症在备注部分，但解释混淆了我。
MSDN actually warns about this gotcha in the remarks section, but the explanation confuses me.
为什么C＃编译器决定，文字0参数应转换为 OdbcType ，而不是对象？该警告还表示，使用 Convert.ToInt32（0）强制对象超负荷使用。
Why does the C# compiler decide that a literal 0 parameter should be converted to OdbcType rather than Object? The warning also says to use Convert.ToInt32(0) to force the Object overload to be used.
据混淆的说，这种转换0到一个对象类型。但不为0已经是对象类型？文字价值观的类型本页面节似乎是说文字是永远类型和 System.Object的所以继承。
It confusingly says that this converts the 0 to an "Object type". But isn't 0 already an "Object type"? The Types of Literal Values section of this page seems to say literals are always typed and so inherit from System.Object.
此行为似乎不是很直观。这是与魂斗罗方差或协变可能？
This behavior doesn't seem very intuitive. Is this something to do with Contra-variance or Co-variance maybe?
推荐答案
按照<一个href="http://www.microsoft.com/downloads/details.aspx?FamilyID=dfbf523c-f98c-4804-afbd-459e846b268e&displaylang=en"相对=nofollow> C＃语言规范4.0 ：
文字0隐式转换为 任何枚举类型。
the literal 0 implicitly converts to any enum type.
因此，的SqlParameter（参数名称，0）解析为的SqlParameter（字符串，OdbcType）过载。
So SqlParameter("parameterName", 0) resolves to the SqlParameter(string, OdbcType) overload.
如果您更改为的SqlParameter（参数名称，1）它解析为的SqlParameter（字符串对象）超载。同样的逻辑也适用于 Convert.ToInt32 。
If you change to SqlParameter("parameterName", 1) it resolves to the SqlParameter(string, object) overload. The same logic applies to Convert.ToInt32.
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