How to Code in C#/.NET Using AI

In short: say the .NET version, use decimal for money, give the method signature and rules, ask for xUnit tests, run dotnet test.

Why C# with AI

C# runs a lot of business software: banking, insurance, retail and government apps. It has strong types and great test tools, so AI mistakes show up fast at build time.

What you need

• .NET SDK 10: the current long-term support version. Check with dotnet --version.
• xUnit: a popular .NET test library. The xunit project template sets it up.
• IDE + assistant: Visual Studio with Copilot, VS Code with C# Dev Kit + Copilot or Claude Code, or Rider.

How it flows

Install and set up

1. Install the .NET 10 SDK from dotnet.microsoft.com (or with the Visual Studio Installer).
2. Check: dotnet --version shows 10.x.
3. Create the projects:

dotnet new classlib -n Billing
dotnet new xunit -n Billing.Tests
dotnet add Billing.Tests reference Billing

Step 1 — the prompt

Step 2 — the code (tested here)

// GST calculator. Built with an AI assistant, then tested.
namespace Billing;

public static class Gst
{
    /// <summary>Returns (gst, total) for a price before tax. Rounds to 2 decimals.</summary>
    public static (decimal Gst, decimal Total) Add(decimal price, decimal ratePercent)
    {
        if (price < 0)
            throw new ArgumentOutOfRangeException(nameof(price), "Price cannot be negative.");
        if (ratePercent < 0 || ratePercent > 100)
            throw new ArgumentOutOfRangeException(nameof(ratePercent), "Rate must be 0 to 100.");

        decimal gst = Math.Round(price * ratePercent / 100m, 2, MidpointRounding.AwayFromZero);
        return (gst, price + gst);
    }
}

Step 3 — ask for xUnit tests, then run them

using Billing;
using Xunit;

public class GstTests
{
    [Fact]
    public void Adds18Percent()
    {
        var (gst, total) = Gst.Add(1000m, 18m);
        Assert.Equal(180m, gst);
        Assert.Equal(1180m, total);
    }

    [Fact]
    public void RoundsToTwoDecimals() => Assert.Equal(5.01m, Gst.Add(100.25m, 5m).Gst);

    [Fact]
    public void RejectsBadRate() =>
        Assert.Throws<ArgumentOutOfRangeException>(() => Gst.Add(100m, 150m));

    [Fact]
    public void RejectsNegativePrice() =>
        Assert.Throws<ArgumentOutOfRangeException>(() => Gst.Add(-1m, 5m));
}

dotnet test
# -> Passed!  - Failed: 0, Passed: 4

Settings and config

1. Turn on nullable checks: <Nullable>enable</Nullable> in the .csproj (new templates do this). It catches AI null mistakes.
2. Treat warnings as errors in CI: dotnet build -warnaserror.
3. Put your rules (".NET 10, xUnit, decimal for money") in .github/copilot-instructions.md or CLAUDE.md.

Good C# prompts (reuse these)

1. “Convert this class to a record with init-only properties.”
2. “Make this method async and pass a CancellationToken.”
3. “Add ILogger logging, but never log card numbers or passwords.”
4. “Explain this exception and fix it:” + the top lines of the stack trace.

Watch out (common AI mistakes in C#)

Quick quiz

Save tokens and money

Paste one class, not the whole solution or bin/obj folders. Paste only the first lines of a stack trace. Let the IDE do free refactors (rename, extract method) and ask the AI only for logic and tests.

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