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If an AI assistant’s C# looks like JavaScript, give it the project’s actual constraints: target framework and language version, nearby C# examples, naming and nullability rules, and the specific behavior the change must implement. Then check the result against the project’s build, formatter, and analyzers. These eight rules make those expectations explicit; they improve guidance but cannot guarantee a correct or buildable answer.
Why does AI write C# like JavaScript?
An assistant responds to the request and to the codebase context available to it. Project instructions and existing files can provide conventions and examples; Microsoft’s overview of how coding agents use technology context describes how agents may draw on what they observe to select platform-specific code and configuration (Microsoft for Developers). GitHub likewise documents repository custom instructions as a way to guide Copilot responses (GitHub Docs).
A reasonable explanation—not a proven cause in every case—is that a broad prompt or mixed-language examples leave room for generic patterns that do not fit C#. The available documentation does not establish a universal cause, measure how often this happens, or show that JavaScript is more represented in assistant training data. The practical remedy is to make the target, local conventions, and validation requirements visible.
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1. Name the language, framework, and target
Say that the change must be C# for this .NET project. Include the target framework and C# language version when known, and ask the assistant not to use syntax or APIs unavailable to them. Check the project settings rather than relying on a guessed default: the C# Guide covers language versions and compatibility.
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For example: “Write C# for this project’s target framework and configured language version. Do not introduce APIs or syntax that those settings do not support.”
2. Make existing code the style authority
Ask the assistant to inspect nearby C# files and the repository’s .editorconfig before proposing a pattern. Existing code may reveal conventions that a generic instruction misses. Keep repository guidance short and specific, and point to established project patterns or documentation where useful; GitHub’s customization guidance describes repository instructions for shaping responses.
3. State the naming conventions
Specify the project’s naming rules instead of assuming the assistant will infer them. A common Microsoft convention uses PascalCase for types and public members, camelCase for parameters and local variables, and a consistent convention for private fields. These are conventions, not C# syntax requirements; the repository’s configured rules take precedence. See Microsoft’s identifier naming conventions and .NET naming rules.
4. Ask for C# structures where they fit
Request appropriate C# types, properties, methods, interfaces, or LINQ rather than constructs that belong to another language. Treat these as tools, not a checklist: a simple function or existing design may be the clearest fit. Microsoft’s C# coding conventions emphasize clarity and simplicity, and describe LINQ as a common tool for working with collections.
5. State the nullable contract
Tell the assistant to preserve the project’s nullable setting and use nullable annotations only when null is part of the intended contract. Ask it to handle nullable values rather than reflexively suppressing warnings. Nullable reference types provide annotations and static analysis; Microsoft notes that “The nullable annotations don’t change the runtime behavior.” Read the nullable reference types documentation.
6. Use async for I/O-bound work
When the operation waits on I/O, ask for appropriate async/await usage and Task return types, following the project’s method-naming convention. Do not make synchronous CPU-bound work asynchronous without a reason. Microsoft explains the async keyword and discusses asynchronous programming scenarios, including I/O-bound operations.
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7. Keep the change small and complete
Define the requested behavior and relevant failure cases, then ask for the smallest change that fits the existing project structure. Explicitly rule out unrelated scaffolding. This gives the assistant a clearer boundary and aligns with Microsoft’s guidance to favor simple, clear code over convoluted logic in its C# coding conventions.
8. Let the build and analyzers check the result
Ask for code that fits the project and for any unresolved build or analyzer checks to be called out. Then run the repository’s normal build, formatter, and configured analyzers. Instructions are advisory; project settings and tooling can surface or enforce some rules. Microsoft documents .NET code-style rules and how to configure C# conventions.
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Adapt this block to the project’s real settings and conventions before using it:
For code in this repository, write idiomatic C# that matches the target framework, configured language version, nearby files, and
.editorconfig. Inspect relevant existing code before creating a new pattern. Follow the repository’s naming and formatting conventions. Preserve its nullable settings; handle nullability warnings rather than suppressing them without explanation. Useasync/awaitfor I/O-bound operations and follow existing asynchronous naming. Prefer clear C# constructs over syntax from other languages. Keep changes limited to the requested task and handle the failure cases stated in the request. Before presenting code, check consistency with the project and identify any build, formatter, or analyzer checks that remain.
This is a starting point, not a guarantee that every assistant product automatically reads repository files. Instruction mechanisms and availability differ by product and version. GitHub documents its Copilot response customization, while Visual Studio documents customizing chat responses. Confirm that the mechanism you use actually supplies the instructions to the assistant.
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Which rules belong in prompts, repository instructions, or tooling?
Use the enforcement method that matches the rule. Prompts are useful for a one-off task; repository-wide instructions help communicate recurring conventions; file-specific guidance can be useful when a product supports it and a particular area needs different context. Compiler diagnostics and configured analyzers provide a stronger check for rules they can detect. In every case, the instruction must fit the project’s framework, language version, and established design, and should be updated as those change.
GitHub and Visual Studio document different instruction mechanisms, so exact behavior depends on the product and version. Neither a prompt nor an instruction file substitutes for compiling and checking the actual project.
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