Fixed: "error IDE0044: Make field readonly" in a .NET C# Build

error IDE0044: Make field readonly appeared the first time I made the build check code style. The rule is right more often than not: the field was set once and never changed, so the compiler can promise that it never will. Here's what the rule checks, why it was an error rather than a hint, and the fix.

What I Was Doing

In a .NET 10 solution with TreatWarningsAsErrors on and the default .editorconfig from dotnet new editorconfig, I set EnforceCodeStyleInBuild to true in Directory.Build.props, so dotnet build checks the code-style rules as well. The class library had this class:

src/Dewiride.Inventory.Data/Class1.cs

namespace Dewiride.Inventory.Data;

public class Class1
{
    private string _prefix = "INV-";

    public required string Name { get; set; }

    public string Code => _prefix + Name;
}

What I Expected

The same clean build as a moment before.

What I Got

Output (dotnet build)

Restore complete (0.6s)
  Dewiride.Inventory.Data net10.0 failed with 1 error(s) (0.1s)
    /Users/you/projects/dewiride-inventory-api/src/Dewiride.Inventory.Data/Class1.cs(5,20): error IDE0044: Make field readonly (https://learn.microsoft.com/dotnet/fundamentals/code-analysis/style-rules/ide0044)

Build failed with 1 error(s) in 0.8s

(5,20) is line 5, column 20: the field _prefix.

What I Checked

What the rule checks. The IDE0044 page: "This style rule concerns specifying the readonly (C#) or ReadOnly (Visual Basic) modifier for private fields that are initialized (either inline or inside of a constructor) but never reassigned." _prefix is private, initialized inline with "INV-", and only read.

The name. The docs call the rule "Add readonly modifier"; the build prints "Make field readonly". Same rule, IDE0044.

Why It Happens

_prefix matches what IDE0044 looks for: a private field, set where it's declared and never assigned again. Three settings together turned that into a failed build:

• The default .editorconfig has dotnet_style_readonly_field = true:warning, the option behind IDE0044, at warning.
• EnforceCodeStyleInBuild made the build run code-style rules, which it skips by default.
• TreatWarningsAsErrors turned the warning into an error.

The code hadn't changed; the build had started looking.

The Fix

Mark the field readonly:

src/Dewiride.Inventory.Data/Class1.cs (the field)

    private readonly string _prefix = "INV-";

readonly means the field can be assigned only where it's declared or in a constructor. The compiler then rejects any later assignment, so nobody can change the prefix by accident.

After the Fix

Output (dotnet build)

Restore complete (0.5s)
  Dewiride.Inventory.Data net10.0 succeeded (0.2s) → src/Dewiride.Inventory.Data/bin/Debug/net10.0/Dewiride.Inventory.Data.dll
  Dewiride.Inventory.Api net10.0 succeeded (0.4s) → src/Dewiride.Inventory.Api/bin/Debug/net10.0/Dewiride.Inventory.Api.dll

Build succeeded in 1.2s

If You Disagree with the Rule

The severity lives in .editorconfig, after the colon. To keep the editor hint without failing the build, lower it:

.editorconfig (under [*.{cs,vb}])

dotnet_style_readonly_field = true:suggestion

suggestion "Violations appear as build messages and as suggestions in the Visual Studio IDE", which doesn't fail a build. Lowering a rule for the whole solution because of one field is rarely right, though; if one field really must stay writable, it'll be because code assigns it, and then the rule doesn't fire.

Common Mistakes

Mistake	What happens	Do this instead
Adding readonly to a field that a method assigns	error CS0191: A readonly field cannot be assigned to (except in a constructor or init-only setter of the type in which the field is defined or a variable initializer)	Leave that field writable; IDE0044 doesn't flag fields that are reassigned
Turning off EnforceCodeStyleInBuild to get rid of the error	Every code-style rule stops running on build	Fix the field
Confusing it with IDE0062 or CA rules	Different rules, different fixes	Read the ID in the message

Troubleshooting

Symptom	Likely cause	Fix
error IDE0044: Make field readonly	A private field is set once and never reassigned, and the build checks code style	Add readonly
IDE0044 shows up only after EnforceCodeStyleInBuild was turned on	The build runs code-style rules only with that setting	Fix the field, or lower the rule's severity in .editorconfig
error CS0191: A readonly field cannot be assigned to … after adding readonly	Code assigns the field after construction	Remove readonly from that field
It's an error, not a warning	TreatWarningsAsErrors is on	Fix the field, as with any warning

FAQ

Does readonly make the code faster?

That's not the point of the rule; it's about intent. A reader sees at once that the value never changes, and the compiler enforces it.

Why didn't the editor and the build agree before?

Code-style rules show in the editor, but command-line builds skip them by default. EnforceCodeStyleInBuild makes the build agree with the editor.

Conclusion

error IDE0044: Make field readonly points at a private field that's set once and never reassigned. Add readonly to it. It became a build error here because the default .editorconfig sets the rule to warning, EnforceCodeStyleInBuild made the build check it, and TreatWarningsAsErrors turned the warning into an error.

Additional Resources

• Add readonly modifier (IDE0044)
• Code analysis rule severity levels

Related

• How to Enforce .NET C# Code Style Rules on Build with EnforceCodeStyleInBuild
• How to Add an .editorconfig to a .NET C# Project with dotnet new editorconfig