c# - c# 表达式上的奇怪类型转换

Question

让我们有一个简单的类

class User
{
   public byte IdByte { get; set; }
}

所以当我写这样的表达式时:

Expression<Func<User, bool>> expression1 = x => x.IdByte == 3;

Expression<Func<User, bool>> expression2 = x => x.IdByte == (byte)3;

byte b = 3;
Expression<Func<User, bool>> expression3 = x => x.IdByte == b;

Expression<Func<User, bool>> expression4 = x => x.IdByte == byte.MaxValue;

然后观察表达式调试 View ，我看到有一个额外的类型转换类型 System.Int32 :

//expression1  ------>  x => (Convert(x.IdByte, Int32) == 3)
//expression2  ------>  x => (Convert(x.IdByte, Int32) == 3)
//expression3  ------>  x => (Convert(x.IdByte, Int32) == Convert(value(....c__DisplayClass1_0).b, Int32))
//expression4  ------>  x => (Convert(x.IdByte, Int32) == 255)

在第一个和第二个表达式中，将右侧 - 3 转换为 byte 比将左侧转换为 int 更合理。
在其余情况下，左侧和右侧是字节
我的问题是为什么应用这些转换？

Answer 1

让我们检查一下 the spec :

Integer comparison operators

The predefined integer comparison operators are:

bool operator ==(int x, int y);
bool operator ==(uint x, uint y);
bool operator ==(long x, long y);
bool operator ==(ulong x, ulong y);

bool operator !=(int x, int y);
bool operator !=(uint x, uint y);
bool operator !=(long x, long y);
bool operator !=(ulong x, ulong y);

bool operator <(int x, int y);
bool operator <(uint x, uint y);
bool operator <(long x, long y);
bool operator <(ulong x, ulong y);

bool operator >(int x, int y);
bool operator >(uint x, uint y);
bool operator >(long x, long y);
bool operator >(ulong x, ulong y);

bool operator <=(int x, int y);
bool operator <=(uint x, uint y);
bool operator <=(long x, long y);
bool operator <=(ulong x, ulong y);

bool operator >=(int x, int y);
bool operator >=(uint x, uint y);
bool operator >=(long x, long y);
bool operator >=(ulong x, ulong y);

Each of these operators compares the numeric values of the two integer operands and returns a bool value that indicates whether the particular relation is true or false.

如您所见，我们实际上并没有比较两个 bytes 或两个 sbytes 的 == 运算符，chars、shorts 或 ushorts。

但是，从这些类型到 int 存在隐式转换，因此编译器将这些转换应用于双方，并使用 bool ==(int x, int y).

如果你写:

byte val = 3;
bool b = val == (byte)4;

编译器会有效地将其转换为:

byte val = 3;
bool b = (int)val == (int)(byte)4;

(int)(byte)4 转换当然是完全没有必要的:编译器只会将其转换为整数文字。

byte val = 3;
bool b = (int)val == 4;