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What are the Visual Studio Debugger's 'Magic Names' and How Do They Work?

Susan Sarandon

Jan 13, 2025 am 11:39 AM

What are the Visual Studio Debugger's

Unveiling the Visual Studio Debugger's Enigmatic Naming Conventions

Visual Studio's debugging prowess extends beyond the surface, employing compiler-generated "magic names" to manage various program entities. These names, while often hidden from the user interface, enable advanced debugging features and optimizations. Understanding these naming patterns is key to effectively navigating the debugger's inner workings.

While Microsoft doesn't publicly document the full implementation details, analysis of the C# GeneratedNames.cs source reveals valuable insights into these naming conventions. Note that these conventions are subject to change without notice.

Hidden Variables and Temporary Storage:

Unused Locals: The debugger tags unused local variables with the __Deleted$ suffix, a clear indicator of compiler optimization.
Temporary Variables: Temporary variables utilize the CS$X$Y naming scheme:
- X: Represents the temporary's nature (e.g., 0 for short-lived, 1 for return values, 2 for lock statements, etc.). A full key is provided below.
- Y: A sequential number identifying the specific temporary instance.

Compiler-Generated Special Names:

The compiler generates unique names for a variety of internal structures and operations, including:

Iterator state (state)
Iterator current value (current)
Iterator parameters and hoisted variables
Iterator closure class instance (locals)
Cached anonymous method delegate (CachedAnonymousMethodDelegate)
Iterator instance (iterator)
Anonymous methods and their closure classes (DisplayClass)
Iterator class
Fixed buffers (FixedBuffer)
Anonymous types (AnonymousType, Field, TPar)
Auto-property backing fields (BackingField)
Iterator thread ID
Iterator finally blocks (Finally)
Dynamic containers and call sites (SiteContainer, Site, SiteDelegate)
COM reference call locals (ComRefCallLocal)
Lock acquisition tracking (LockTaken)
Query expression temporaries (TransparentIdentifier)

The Magic Name Formula:

The general pattern for these magic names follows: P<n>C__SI</n>, where:

P: CS$ prefix for cached delegates and display class instances; otherwise, empty.
<n></n>: The original variable name (if applicable).
C: A single character (1 to s) indicating the entity type.
S: An optional descriptive suffix.
I: An optional unique identifier.

Temporary Variable Kind Key:

0: Short-lived temporaries
1: Return value temporaries
2: Lock statement temporaries
3: Using statement temporaries
4: Durable temporaries
5: Foreach enumerator result
6: Foreach array storage
7: Foreach array index storage
8-264: Multidimensional array index storages
264: String fixed statement temporaries

This detailed breakdown offers a clearer understanding of the often-cryptic names encountered while debugging in Visual Studio. Remember that this information is based on observation and is subject to change with future compiler updates.

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