Run C# Files Directly Without Project Setup
Microsoft’s .NET 10 Preview 4 introduces a feature that fundamentally changes how developers interact with C#. For the first time in .NET’s history, you can execute C# files directly using dotnet run app.cs without creating project files, scaffolding directories, or managing MSBuild configurations.
This is a paradigm shift that positions C# alongside scripting languages while maintaining enterprise-grade capabilities.
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✔️As Per the Release Notes
Until .NET 10 Preview 4, executing C# code through the dotnet CLI mandated a project structure containing a .csproj file. This requirement created friction between developers and their ideas, forcing overhead that often exceeded the actual code being written. The new file-based apps capability eliminates this barrier entirely.
The implementation works by creating an implicit project file behind the scenes. When you execute dotnet run hello.cs, the .NET runtime generates the necessary project structure in memory, compiles your code, and executes it without persisting any additional files to disk.
This approach maintains full compatibility with the existing .NET ecosystem while providing the immediacy developers expect from modern development tools.
The feature operates on the same C# compiler, uses identical language syntax, and provides access to the complete .NET ecosystem. This ensures that code written as file-based apps can seamlessly transition to full projects without modification or compatibility concerns.
✔️File-Level Directives: Power Without Complexity
.NET 10 introduces file-level directives that embed project-like capabilities directly within .cs files. These directives solve the fundamental challenge of single-file development: accessing external dependencies and configuration without sacrificing simplicity.
Package Management with #:package
The #:package directive enables NuGet package references directly within your source file:
#:package Humanizer@2.14.1
using Humanizer;
var dotNet9Released = DateTimeOffset.Parse("2024-12-03");
var since = DateTimeOffset.Now - dotNet9Released;
Console.WriteLine($"It has been {since.Humanize()} since .NET 9 was released.");This directive eliminates the package management ceremony typically associated with .NET development. Developers can experiment with libraries, build prototypes, and create utilities without navigating package manager interfaces or editing project files.
SDK Specification with #:sdk
Different application types require different SDKs. The #:sdk directive allows file-based apps to specify their target SDK:
#:sdk Microsoft.NET.Sdk.Web
var builder = WebApplication.CreateBuilder();
builder.AddOpenApi();
var app = builder.Build();
app.MapGet("/", () => "Hello, world!");
app.Run();This directive helps in the full capabilities of specialized SDKs. Web applications gain access to ASP.NET Core features, including Minimal APIs, dependency injection, and middleware pipelines. The directive system ensures that file-based apps aren’t limited to console applications.
MSBuild Properties with #:property
The #:property directive provides access to MSBuild configuration:
#:property LangVersion preview
// Access to preview language features
string message = "Advanced C# features enabled";
Console.WriteLine(message);This capability enables developers to configure language versions, target frameworks, and build behaviors without creating project files. Advanced scenarios that previously required MSBuild knowledge become accessible through simple directive syntax.
Cross-Platform Scripting with Shebang Support
File-based apps support shebang lines, enabling C# to function as a first-class scripting language on Unix-like systems:
#!/usr/bin/dotnet run
Console.WriteLine("Hello from a C# script!");After making the file executable with chmod +x app.cs, you can execute it directly: ./app.cs. This functionality positions C# as a viable option for system administration, DevOps automation, and cross-platform utilities.
The shebang support addresses a significant gap in the .NET ecosystem. Previously, C# required substantial project overhead for simple automation tasks. Now, C# scripts can compete directly with Python, Bash, and PowerShell for operational workflows.
✔️Production Considerations and Use Case
Development and Prototyping
File-based apps excel in scenarios requiring rapid iteration. Algorithm development, API experimentation, and proof-of-concept implementations benefit from reduced startup friction. The ability to test ideas immediately without project setup encourages experimentation and innovation.
Educational environments gain significant advantages from this feature. Instructors can focus on language concepts and problem-solving rather than project management. Students encounter fewer barriers when learning C# fundamentals, improving learning outcomes and reducing frustration.
Automation and Scripting
System administrators and DevOps engineers can leverage C# for infrastructure automation. The combination of .NET’s robust libraries, strong typing, and cross-platform execution creates compelling alternatives to traditional scripting languages.
Build automation, deployment scripts, and monitoring utilities benefit from C#’s ecosystem while maintaining the simplicity expected from scripting languages. The ability to access NuGet packages through file-level directives enables sophisticated automation scenarios without project complexity.
Integration and Data Processing
Data transformation, API integration, and file processing tasks often require more sophisticated capabilities than traditional scripting languages provide. File-based C# apps deliver enterprise-grade libraries and performance while maintaining script-like simplicity.
JSON processing, HTTP client operations, and database connectivity become accessible through simple directive-based package references. This capability bridges the gap between quick scripts and full applications.
Project Conversion and Growth Path
The dotnet project convert app.cs command provides seamless transition from file-based apps to traditional projects. This conversion process creates a new directory, generates appropriate .csproj files, and translates file-level directives into MSBuild equivalents.
Consider this file-based web application:
#:sdk Microsoft.NET.Sdk.Web
#:package Microsoft.AspNetCore.OpenApi@10.*-*
var builder = WebApplication.CreateBuilder();
builder.AddOpenApi();
var app = builder.Build();
app.MapGet("/", () => "Hello, world!");
app.Run();The conversion process generates this project structure:
net10.0
enable
enable
This conversion maintains all functionality while providing access to advanced project features including multiple files, resources, and complex build configurations. The growth path ensures that file-based apps can evolve naturally without rewrites or compatibility issues.
Development Environment Integration
Visual Studio Code provides enhanced support for file-based apps through the C# Dev Kit extension. The pre-release version (2.79.8 or later) includes IntelliSense support for file-level directives, debugging capabilities, and integrated terminal execution.
The development experience maintains parity with traditional project-based development. Syntax highlighting, error detection, and code completion function identically to standard C# development environments.
Future .NET 10 previews will enhance IDE integration with improved performance, enhanced debugging support, and better directive IntelliSense. Microsoft is also exploring multi-file support and execution performance optimizations.
Performance and Architectural Considerations
File-based apps compile and execute using the same infrastructure as traditional .NET applications. Performance characteristics remain identical to project-based applications because the underlying compilation and runtime processes are unchanged.
The implicit project generation adds minimal overhead during startup. For most applications, this overhead is negligible compared to application initialization time. Performance-critical scenarios should measure actual impact rather than assuming overhead significance.
Memory usage patterns mirror traditional applications because file-based apps utilize the same runtime environment. The simplified development model doesn’t compromise execution efficiency or resource utilization.
Ecosystem Integration and Limitations
File-based apps integrate seamlessly with the broader .NET ecosystem. NuGet packages, runtime libraries, and platform APIs function identically to traditional applications. The feature maintains full compatibility with existing .NET investments and knowledge.
Current limitations include support for single files only, though multi-file support is under exploration. Complex project scenarios requiring custom MSBuild targets, resource files, or specialized build processes require traditional project structures.
The directive system covers common scenarios but doesn’t replicate every MSBuild capability. Advanced build customization, conditional compilation beyond basic properties, and complex dependency management may necessitate project conversion.
Competitive Landscape and Strategic Implications
This capability positions C# competitively against Python, Node.js, and other ecosystems that emphasize developer experience and rapid prototyping. The combination of strong typing, comprehensive libraries, and immediate execution creates compelling advantages for many development scenarios.
Educational institutions can now introduce C# without project management complexity. This reduces barriers to adoption and enables focus on programming fundamentals rather than tooling overhead.
Enterprise environments benefit from unified technology stacks. Teams can use C# for both large applications and operational scripts, reducing context switching and leveraging existing expertise across broader use cases.
Implementation Strategy for Development Teams
Organizations should establish guidelines for appropriate file-based app usage. Clear boundaries between single-file scripts and project-worthy applications prevent architectural decisions that become technical debt.
Development teams should consider file-based apps for prototyping, utility creation, and educational scenarios. Production applications requiring multi-file organization, complex build processes, or enterprise deployment patterns should use traditional project structures.
Training programs should incorporate file-based app capabilities to improve developer onboarding and reduce learning curves. The simplified development model can accelerate team productivity for appropriate use cases.
Future Roadmap and Evolution
Microsoft’s roadmap includes enhanced Visual Studio Code integration, multi-file support exploration, and execution performance improvements. The feature will continue evolving based on community feedback and usage patterns.
REPL (Read-Eval-Print Loop) functionality represents a logical extension of file-based capabilities. Interactive C# development could further reduce the friction between developers and their ideas.
Double-click execution on Windows remains unlikely due to security concerns, but future versions running under WASI (WebAssembly System Interface) might enable sandboxed execution scenarios.
The .NET 10 file-based apps feature represents more than a convenience improvement. It fundamentally alters C#’s position in the development ecosystem, making it viable for scenarios previously dominated by scripting languages while maintaining enterprise-grade capabilities.
This feature democratizes C# development by removing barriers that prevented experimentation and learning. The seamless growth path from single files to enterprise applications ensures that investments in file-based development aren’t wasted as requirements evolve.
Every developer working with C# should understand and experiment with this capability. The combination of simplicity and power creates opportunities for innovation, education, and productivity improvements that extend far beyond the immediate convenience of avoiding project files.
The future of C# development just became more accessible, more flexible, and more aligned with modern development expectations. The question isn’t whether to adopt this feature, but how quickly you can integrate it into your development workflows.
