Code analysis in C# string literals

Last modified: 15 May 2025

note

The functionality described below is only available in .NET/.NET Core and ASP.NET Core projects and requires Smart Mode to be enabled.

In system string formatting methods

JetBrains Fleet analyzes format strings and arguments of all .NET string formatting methods, such as String.Format, Text.StringBuilder.AppendFormat, or Console.WriteLine.

In usages of string formatting methods, JetBrains Fleet highlights format placeholders and also synchronously highlights placeholder with the corresponding argument when your caret is at either of them:

Highlighting of arguments and placeholders in string formatting methods

If arguments and format placeholders mismatch (which leads to the FormatException in runtime if arguments are missing), JetBrains Fleet generates warnings for missing or redundant arguments:

Warning for missing arguments in string formatting methods

You can easily fix this problem by pressing over the warning. JetBrains Fleet will suggest either automatically adding the missing argument or to remove the mismatched format placeholder.

JetBrains Fleet also helps you detect and remove redundant calls of string formatting methods inside other string formatting methods. For example:

Redundant call to string formatting method

In custom string formatting methods

To enable code analysis and assistance features in custom string formatting methods, use the [StringFormatMethod] and [StructuredMessageTemplate] attributes from the JetBrains.Annotations namespace.

Consider a custom string formatting method ShowError:

public void ShowError(string formatString, params object[] args)
{
    // some custom logic
    Console.WriteLine(formatString, args);
}

If the method is called incorrectly, as shown below, JetBrains Fleet has no chance to detect the missing argument:

String formatting method called incorrectly

Make JetBrains Fleet aware of a custom string formatting method

  1. Reference the JetBrains.Annotations namespace as described in the Annotations in C# source code section.

  2. Annotate your custom string formatting method with the [StringFormatMethodAttribute] attribute, which takes a single argument — the name of the format string parameter:

    [StringFormatMethod("formatString")]
public void ShowError(string formatString, params object[] args)
{
    // some custom logic
    Console.WriteLine(formatString, args);
}

    JetBrains Fleet will be able to warn you about missing arguments when this custom formatting method is invoked.

    Quick-fix for the string formatting method call

  3. Alternatively, annotate the parameter that accepts a formatted string with the [StructuredMessageTemplateAttribute]:

    void LogNewUser([StructuredMessageTemplate] string message, params string[] args)
{
    // Log new user
}

void Test()
{
    // Warning: Non-existing argument in format string
    LogNewUser("User created: {username}");
}

    This second approach allows you to use custom strings as placeholders, as username in the example above.

tip

You can annotate custom string formatting methods in compiled libraries using external annotations for c#.

In interpolated strings

Modern versions of C# provide a more elegant alternative to the String.Format method — interpolated strings. That's why JetBrains Fleet highlights usages of String.Format with the suggestion to convert them into interpolated strings. You can use a quick-fix to apply the conversion with a couple of keystrokes.

JetBrains Fleet: Converting a usage of String.Format to string interpolation

If you need to quickly add the $ at the beginning of the string when you are deep into typing out the string, you can press and choose To string interpolation.

JetBrains Fleet: Converting simple string to string interpolation

See also

Concepts