如何在 C# 中从 MySQL 检索 tinyint 数据类型？

Question

So in C# whenever I retrieved a tinyint from my MSSQL database I used the following cast.

(int)(byte)reader["MyField"];

However, that cast doesn't seem to work in MySQL.

What I have tried

(byte)reader["MyField"];

and just

(int)reader["MyField"];

Edit 1

Exception

The specified cast is not valid.

Edit 2

This is the data type.

{Name = "SByte" FullName = "System.SByte"}

Answer 1

To determine the proper type, look at the value of

reader["MyField"].GetType()

in the debugger.

Answer 2

The problem is that due to casting and explicit operators:

(byte)objectExpression is is not the same as (byte)sbyteExpression .

The first is a [direct] cast which fails because the real object type is sbyte and not byte . The latter will perform a conversion that just happens to use a explicit operator (an "Explicit Conversion") with syntax that, unfortunately, still looks like a [direct] cast as per above. Here is an example of it failing sans-database:

var obj = (object)(sbyte)0;
var i1 = (int)(sbyte)obj;  // okay: object (cast)-> sbyte (conversion)-> int
var i2 = (int)obj;         // fail: sbyte (cast)-> int (but sbyte is not int!)

Either use an (sbyte)objectExpression cast which is valid for the real object type, or Convert.ToInt32(objectExpression) which takes an object and does some magic to convert it to an int. (Using Convert.ToByte could throw an exception on overflow.)

Happy coding!

Answer 3

May I suggest to let the system work against itself? The DataReader class provides features for getting the correct type of value:

reader.GetInt32("id");
reader.GetByte("MyByteField");

This way the Reader provides you with the type you expect.