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A constant is a symbol that has a never-changing value . When defining a constant symbol, its value must be determinable at compile time . The compiler then saves the constant s value in the assembly s metadata . This means that you can define a constant only for types that your compiler considers primitive types . In C#, the following types are primitives and can be used to define constants: Boolean, Char, Byte, SByte, Int16, UInt16, Int32, UInt32, Int64, UInt64, Single, Double, Decimal, and String . However, C# also allows you to define a constant variable of a non-primitive type if you set the value to null:
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using System; public sealed class SomeType { // SomeType is not a primitive type but C# does allow // a constant variable of this type to be set to 'null'. public const SomeType Empty = null; }
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Because a constant value never changes, constants are always considered to be part of the defining type . In other words, constants are always considered to be static members, not instance members . Defining a constant causes the creation of metadata . When code refers to a constant symbol, compilers look up the symbol in the metadata of the assembly that defines the constant, extract the constant s value, and embed the value in the emitted Intermediate Language (IL) code . Because a constant s value is embedded directly in code, constants don t require any memory to be allocated for them at runtime . In addition, you can t get the address of a constant and you can t pass a constant by reference . These constraints also mean that constants don t have a good cross-assembly versioning story, so you should use them only when you know that the value of a symbol will never change .
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(Defining MaxInt16 as 32767 is a good example .) Let me demonstrate exactly what I mean . First, take the following code and compile it into a DLL assembly:
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using System; public sealed class SomeLibraryType { // NOTE: C# doesn't allow you to specify static for constants // because constants are always implicitly static. public const Int32 MaxEntriesInList = 50; }
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Then use the following code to build an application assembly:
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using System; public sealed class Program { public static void Main() { Console.WriteLine("Max entries supported in list: " + SomeLibraryType.MaxEntriesInList); } }
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You ll notice that this application code references the MaxEntriesInList constant defined in the SomeLibraryType class . When the compiler builds the application code, it sees that MaxEntriesInList is a constant literal with a value of 50 and embeds the Int32 value of 50 right inside the application s IL code, as you can see in the IL code shown below . In fact, after building the application assembly, the DLL assembly isn t even loaded at runtime and can be deleted from the disk .
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.method public hidebysig static void Main() cil managed { .entrypoint // Code size 25 (0x19) .maxstack 8 IL_0000: nop IL_0001: ldstr "Max entries supported in list: " IL_0006: ldc.i4.s 50 IL_0008: box [mscorlib]System.Int32 IL_000d: call string [mscorlib]System.String::Concat(object, object) IL_0012: call void [mscorlib]System.Console::WriteLine(string) IL_0017: nop IL_0018: ret } // end of method Program::Main
This example should make the versioning problem obvious to you . If the developer changes the MaxEntriesInList constant to 1000 and only rebuilds the DLL assembly, the application assembly is not affected . For the application to pick up the new value, it will have to be recompiled as well . You can t use constants if you need to have a value in one assembly picked up by another assembly at runtime (instead of compile time) . Instead, you can use readonly fields, which I ll discuss next .
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