> ## Documentation Index
> Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt
> Use this file to discover all available pages before exploring further.
# C# File-Scoped Namespace
A file-scoped namespace is a C# 10 language feature that applies a namespace declaration to all types within a single source file without requiring enclosing braces. By terminating the namespace declaration with a semicolon rather than a code block, it alters the lexical structure of the file and eliminates an indentation level for all subsequent type declarations.
## Syntax Comparison
**Traditional Block-Scoped Namespace:**
```csharp theme={"dark"}
namespace Company.Project.Module
{
public class Processor
{
// Indented code
}
}
```
**File-Scoped Namespace:**
```csharp theme={"dark"}
namespace Company.Project.Module;
public class Processor
{
// Unindented code
}
```
## Compiler Rules and Constraints
The C# compiler enforces strict structural rules when parsing file-scoped namespaces:
1. **Single Declaration Limit:** A source file can contain a maximum of one file-scoped namespace declaration.
2. **Mutual Exclusivity:** A file cannot mix file-scoped namespaces and block-scoped namespaces. If a file-scoped namespace is declared, no `namespace { ... }` blocks are permitted in that same file.
3. **Placement Precedence:** The file-scoped namespace declaration must precede all type declarations (classes, structs, interfaces, enums, delegates) in the source file.
4. **No Sequential Nesting:** You cannot declare multiple file-scoped namespaces sequentially to achieve nesting.
**Invalid:**
```csharp theme={"dark"}
namespace Company;
namespace Project; // Compiler error: Source file can only contain one file-scoped namespace
```
**Valid:**
```csharp theme={"dark"}
namespace Company.Project; // Use dot notation for nesting
```
## Interaction with `using` Directives
`using` directives can be placed either before or after a file-scoped namespace declaration. Both placements are syntactically valid, though they carry the same scoping implications as they would with a block-scoped namespace.
```csharp theme={"dark"}
using System; // Valid
namespace Company.Project.Module;
using System.Linq; // Valid
public class Processor { }
```
## Intermediate Language (IL) Equivalence
File-scoped namespaces are purely syntactic sugar. During compilation, the Roslyn compiler translates a file-scoped namespace into the exact same Intermediate Language (IL) as a block-scoped namespace. There is no runtime difference, metadata difference, or reflection difference between types declared using either syntax.
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