The C# compiler normally emits a .localsinit flag that forces local variables and stacks to be zero-initialized by the CLR.

This is usually redundant as C# uses definite assignment in all but a few unsafe scenarios (covered below).

In C# 9.0 the SkipLocalsInitAttribute may be used to suppress this flag for some small performance gains. Applying it to a method affects all that methods local variables including any local functions or lambdas it declares. If applied to a type it applies to all methods on that type, when applied to a module then to all methods in that assembly.

Code

Now

[SkipLocalsInit]
static void ProcessBuffer()
{
    Span<byte> buffer = stackalloc byte[256];
    // buffer contents are NOT zero-initialized — slightly faster
    Fill(buffer);
    Send(buffer);
}

Before

static void ProcessBuffer()
{
    Span<byte> buffer = stackalloc byte[256];
    // CLR always zero-initializes the buffer before use
    Fill(buffer);
    Send(buffer);
}

Notes

• Care must be taken for the scenarios in which C# does not enforce definite assignment and might contain uninitialized data. They are unsafe code, stackalloc usages, and P/Invoke scenarios

More information

• Specification
• Fun with SkipLocalsInit @ Dru's Delirium
• Improving performance using SkipLocalsInit @ Meziantou's Blog
• ASP.NET team discussing using SkipLocalsInit @ GitHub