定义：
扩大转换：当一个类型的值转换为相等或更大范围的另一种类型时
缩小转换：与扩大转换相反
Details
扩大转换
Byte->Uint16,Int16,Uint32,Int32,UInt64,Single,Double,Decimal
SByte->Int16,Int32,Int64,Single,Double,Decimal
Int16->Int32,Int64,UInt64,Int64,Single,Double,Decimal
Char->UInt64,UInt32,Int32,UInt64,Int64,Single,Double,Decimal
Int32->Int64,Double,Decimal
UInt32->Int64,UInt64,Double,Decimal
Int64->Decimal
UInt64->Decimal
Single->Double