Struct initialization and default values – definitive assignment place

[bravequickcleverfibreyarn]

Type of issue

Typo

Description

From https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/struct#struct-initialization-and-default-values

The compiler performs its usual definite assignment analysis. Any fields that are accessed before being assigned, or not definitely assigned when the constructor finishes executing are assigned their default values before the constructor body executes.

This phrase is little confusing. Having example

internal struct FieldInitializationTestStruct
{
    public int x;
    public int y = 1;
    public int z;

    public FieldInitializationTestStruct()
    {
        var f = x+2;
        z = x;
    }
}

MSIL produced for .ctor body is

IL_0000: ldarg.0
IL_0001: ldc.i4.0
IL_0002: stfld int32 LangEnvTests.FieldInitializationTestStruct::x // intialized here
IL_0007: ldarg.0
IL_0008: ldc.i4.1
IL_0009: stfld int32 LangEnvTests.FieldInitializationTestStruct::y
IL_000e: ldarg.0
IL_000f: ldarg.0
IL_0010: ldfld int32 LangEnvTests.FieldInitializationTestStruct::x
IL_0015: stfld int32 LangEnvTests.FieldInitializationTestStruct::z
IL_001a: ret

so field is initialized before access but in constructor body.

So, IMHO:

• Having default struct, all fields are initialized by default to 0-bit pattern via initobj.
• Having new struct, all fields are initialized as part of ctor run where field and default initializations and are place to .ctor by compilation.

Page URL

https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/struct#struct-initialization-and-default-values

Content source URL

https://github.com/dotnet/docs/blob/main/docs/csharp/language-reference/builtin-types/struct.md

Document Version Independent Id

0960802e-80c3-cb56-cb8b-ab1f1b20fb4e

Article author

@BillWagner

Metadata

• ID: d9220116-e950-04cf-b6d5-70d15a084a22
• Service: dotnet-csharp
• Sub-service: lang-reference