Decimal.MinValue & Decimal.MaxValue: why static readonly and not const modifiers?

问题

In C#, MinValue field is defined for numeric types with:

① static readonly modifiers for decimal type (Link to MSDN Libray for .NET 4.5):

public static readonly decimal MinValue

② const modifier for all other numeric types:

//Integral signed numeric types
public const sbyte MinValue
public const short MinValue
public const int MinValue
public const long MinValue

//Integral unsigned numeric types
public const byte MinValue
public const ushort MinValue
public const uint MinValue
public const ulong MinValue

//Real numeric types
public const float MinValue
public const double MinValue

Why const modifier is not used to define Decimal.MinValue field ?

Remarks:

① Same question applies to numeric types MaxValue field.

② VB, C++ and F# also use different modifiers for decimal type so this question is not specific to C# language.

回答1:

This is an interesting question. I did some research, and the MS documentation on Decimal Min/Max fields explicitly says (note the wording is the same for both; shown in brackets for clarity):

The value of this constant is [negative] 79,228,162,514,264,337,593,543,950,335.

The following code compiles with no problem:

public const decimal MaxValue = 79228162514264337593543950335M;

- Edit -

Note: here is the assignment of the field from the source for .NET 4.5 (downloaded PDB source from MS), the decimal constructor that is called by this code. Note that it is declaring a const value. It appears, at least for 4.5, that the documentation is wrong. (This would not be the first time MS documentation is incorrect). It also appears that the source code won't compile, as pointed out in a comment by @Daniel.

public const Decimal MinValue = new Decimal(-1, -1, -1, true, (byte) 0);
public const Decimal MaxValue = new Decimal(-1, -1, -1, false, (byte) 0);

public Decimal(int lo, int mid, int hi, bool isNegative, byte scale)
{
  if ((int) scale > 28)
    throw new ArgumentOutOfRangeException("scale", Environment.GetResourceString("ArgumentOutOfRange_DecimalScale"));
  this.lo = lo;
  this.mid = mid;
  this.hi = hi;
  this.flags = (int) scale << 16;
  if (!isNegative)
    return;
  this.flags |= int.MinValue;
}

Also note: in the 2.0 framework, decimal is declared outright:

public const Decimal MaxValue = 79228162514264337593543950335m;

So, inconsistency and incorrect documentation is the conclusion I have reached. I will leave it to others to look at the other framework versions for a pattern.

回答2:

Although MSDN library describes decimal MinValue field as being static readonly, C# compiler treats it as const.

If MinValue field was readonly, following code would not compile, but it actually does compile.

const decimal test = decimal.MinValue - decimal.MinValue;

Remarks:

① Same answer applies to decimal type MaxValue field.

② For further details, Jon Skeet's gives here an insight on how constant field implementation at IL level differs between:

Primitive numeric types (integrals, float and double) and
The non primitive numeric decimal type.

回答3:

Firstly, note that these Decimal values are seen as constants from the languages' point of view (but not the CLR), as Jon Skeet mentions in his answer to rmayer06's related question.

(I thought the reason for using ReadOnly instead of Const was so that the constructor was not called on every use of a Const, except that is :-( )

If you compile: x == Decimal.MaxValue in C# or VB.NET, the constant is constructed as if it was really a Const.

In VB.NET this is not true for Decimal.One, Decimal.Zero or Decimal.MinusOne, but these are treated as constants in C#. (BTW String.Empty is not seen as a constant in either language.)

So I believe these are baked into to languages as constants (sometimes, in VB's case).

In VB's case it does seem to prefer to load the ReadOnly values, except for MaxValue, and Minvalue. (That is Const y1 As Decimal = Decimal.One is effectively aliased by VB, but truly treated as a constant in C#.)

BTW Date.MaxValue is also ReadOnly, but it is not seen as a constant by VB.NET (even though it does have Date literals).

UPDATE: I haven't checked what the current results are for the most recent VB.NET or C# but the reference source does not treat any of Decimal.One, Decimal.Zero, Decimal.MinusOne, Decimal.MaxValue or Decimal.MinValue specially; they're all just public const.

回答4:

I'd have thought it was for readability of the source code

public const Decimal MaxValue = new Decimal(-1, -1, -1, false, (byte) 0);

seems more readable than the magic number:

public const Decimal MaxValue = 79228162514264337593543950335m

来源：https://stackoverflow.com/questions/21501369/decimal-minvalue-decimal-maxvalue-why-static-readonly-and-not-const-modifiers

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