Fix formatting exception when comparing strings containing braces by Copilot · Pull Request #3151 · fluentassertions/fluentassertions · GitHub
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Fix formatting exception when comparing strings containing braces - #3151

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dennisdoomen merged 3 commits into
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copilot/fix-system-format-exception
Jan 26, 2026
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Fix formatting exception when comparing strings containing braces#3151
dennisdoomen merged 3 commits into
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copilot/fix-system-format-exception

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Copilot AI commented Jan 21, 2026 •
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Be() assertion throws FormatException instead of test failure when comparing strings containing { or } characters.

Root Cause

StringEqualityStrategy.GetMismatchSegmentForLongStrings used StringBuilder.AppendFormat with an interpolated string literal containing user values:

sb.AppendFormat(CultureInfo.InvariantCulture,
    $"""
     Full expectation:
     {expected.RenderAsIndentedBlock().AsNonFormattable()},
     
     Full subject:
     {subject.RenderAsIndentedBlock().AsNonFormattable()}
     """);

This causes double-interpretation: string interpolation inserts user values first, then AppendFormat interprets the result as a format string. Braces in user strings are mistaken for format placeholders.

Changes

  • Use string.Create() to build the interpolated string, then Append() the result
  • Remove unnecessary .AsNonFormattable() calls since we no longer use AppendFormat
string fullDetails = string.Create(CultureInfo.InvariantCulture,
    $"""
     Full expectation:
     {expected.RenderAsIndentedBlock()},
     
     Full subject:
     {subject.RenderAsIndentedBlock()}
     """);
sb.Append(fullDetails);

This ensures user strings are never reinterpreted as format strings.

Original prompt

This section details on the original issue you should resolve

<issue_title>Be() throws System.FormatException when expected value contains "{"</issue_title>
<issue_description>### Description

Hi,

I was writing tests for my code formatter and came across this weird behavior - FluentAssertions use string.Format without sanitizing the values (but that is not optimal solution IMHO).

It is kind of annoying:

This works just fine:

// Ok
[Fact]
public void FormatParameters_MultpleParameters_MovedParametersToTheirOwnLines()
{
    _formatter.Format("""
        codeunit 0 C
        {
            procedure Nop(X: Integer; Y: Integer; Z: Integer)
            begin
            end;
        }
        """).Should().Be("""
        codeunit 0 C
        {
            procedure Nop(
                X: Integer;
                Y: Integer;
                Z: Integer)
            begin
            end;
        }
        """);
}

This throws throws System.FormatException.

[Fact]
public void FormatParameters_MultipleParameters_ShrinksUnnecessaryWhitespace()
{
    _formatter.Format("""
        codeunit 0 C
        {
            procedure Nop(

        // X
        X: Integer;

        // Y
        Y: Integer;

        // Z
        Z: Integer)
            begin
            end;
        }
        """).Should().Be("""
        codeunit 0 C            
        {
            procedure Nop(
                // X
                X: Integer;
                // Y
                Y: Integer;
                // Z
                Z: Integer)
            begin
            end;
        }
        """;
}

Thanks

Reproduction Steps

using FluentAssertions;
using Xunit.Sdk;

namespace DemonCore.Tests;

public class DemonCoreTests
{
    [Fact]
    public void NotOk()
    {
        DemonCore(80).Should().Throw<XunitException>(); // Be() throws System.FormatException
    }

    [Fact]
    public void Ok()
    {
        DemonCore(79).Should().Throw<XunitException>(); // Be() does not throw but fails to format something.
        /*
        Actual assertion message if we just run `DemonCore(79)();`.
    
        **WARNING** failure message 'Expected actual to be a match with the expectation, but it differs at index 0:
           ↓ (actual)
          "{xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
          ""
           ↑ (expected)' could not be formatted with string.Format
           at System.Text.ValueStringBuilder.AppendFormatHelper(IFormatProvider provider, String format, ReadOnlySpan`1 args)
           at System.String.FormatHelper(IFormatProvider provider, String format, ReadOnlySpan`1 args)
           at System.String.Format(IFormatProvider provider, String format, Object[] args)
           at FluentAssertions.Execution.FailureMessageFormatter.FormatArgumentPlaceholders(String failureMessage, Object[] failureArgs)
        */
    }

    private static Action DemonCore(int number) =>
        () =>
        {
            var actual = "{" + new string('x', number);
            var expected = "";
            actual.Should().Be(expected);
        };
}
<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>net10.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
    <IsPackable>false</IsPackable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="coverlet.collector" Version="6.0.4" />
    <PackageReference Include="FluentAssertions" Version="8.8.0" />
    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.0.1" />
    <PackageReference Include="xunit" Version="2.9.3" />
    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.5">
      <PrivateAssets>all</PrivateAssets>
      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
    </PackageReference>
  </ItemGroup>

  <ItemGroup>
    <Using Include="Xunit" />
  </ItemGroup>

</Project>

P.S.: The fact that it fails around 79-80 is funny.

Expected behavior

Be should throw only XunitException for string comparisons - strings are either equal or not equal.

IMHO, building final assertion message incrementally using multiple string.Format calls feels too hacky.
Probably, a better approach would be to create some tree-ish type and walking over it to build the final string. But I'm not regularly programming in C# so don't listen to me. :)

Actual behavior

In reproduction tests:

  • The NotOk test fails - the actual string is "{" and then 80+ "x"s and it is compared to an empty string.
  • The Ok test succeeds but it has string.Format error in the assertion message - the actual string is "{" and 79 "x"s.

Full dotnet test result:

PS C:\MY\NotWork\UpdateAlWorkspaceThing> dotnet test
Restore complete (0,5s)
  UpdateAlWorkspaceThing.Tests net10.0 succeeded (0,1s) → UpdateAlWorkspaceThing.Tests\bin\Debug\net10.0\UpdateAlWorkspaceThing.Tes...

</details>



<!-- START COPILOT CODING AGENT SUFFIX -->

- Fixes fluentassertions/fluentassertions#3147

<!-- START COPILOT CODING AGENT TIPS -->
---

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@dennisdoomen
Fix FormatException when comparing strings with braces
e40f7ef 
Co-authored-by: dennisdoomen <572734+dennisdoomen@users.noreply.github.com>
Copilot AI changed the title [WIP] Fix System.FormatException when value contains "{" Fix FormatException when comparing strings containing braces Jan 21, 2026
Copilot AI requested a review from dennisdoomen January 21, 2026 19:41
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github-actions Bot commented Jan 21, 2026 •
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coveralls commented Jan 21, 2026 •
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Pull Request Test Coverage Report for Build 21300676482

Details

  • 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
  • No unchanged relevant lines lost coverage.
  • Overall coverage increased (+0.001%) to 97.185%
Totals Coverage Status
Change from base Build 20931548150: 0.001%
Covered Lines: 12851
Relevant Lines: 13070
💛 - Coveralls

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github-actions Bot commented Jan 21, 2026 •
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Qodana for .NET

It seems all right 👌

No new problems were found according to the checks applied

💡 Qodana analysis was run in the pull request mode: only the changed files were checked
☁️ View the detailed Qodana report

Contact Qodana team

Contact us at qodana-support@jetbrains.com

@dennisdoomen
dennisdoomen marked this pull request as ready for review January 23, 2026 20:45
@dennisdoomen
Improve test clarity by using named constant
e773f85 
Co-authored-by: dennisdoomen <572734+dennisdoomen@users.noreply.github.com>
@dennisdoomen
dennisdoomen force-pushed the copilot/fix-system-format-exception branch from 233a12c to e773f85 Compare January 23, 2026 20:47
@dennisdoomen
dennisdoomen requested a review from jnyrup January 23, 2026 20:49
@jnyrup jnyrup linked an issue Jan 25, 2026 that may be closed by this pull request
Be() throws System.FormatException when expected value contains "{" #3147
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jnyrup
jnyrup reviewed Jan 25, 2026
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Comment thread Src/FluentAssertions/Primitives/StringEqualityStrategy.cs

@jnyrup jnyrup Jan 25, 2026

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If we polyfill StringBuilder.Append in StringBuilderExtensions we can avoid the intermediate string.

sb.Append(CultureInfo.InvariantCulture,
    $"""


        Full expectation:

        {expected.RenderAsIndentedBlock()},

        Full subject:

        {subject.RenderAsIndentedBlock()}

        """);

For StringBuilderExtensions

public static StringBuilder AppendLine(this StringBuilder stringBuilder, IFormatProvider provider, FormattableString formattable) =>
    stringBuilder.AppendLine(string.Create(provider, formattable));

public static StringBuilder Append(this StringBuilder stringBuilder, IFormatProvider provider, FormattableString formattable) =>
    stringBuilder.Append(string.Create(provider, formattable));

@dennisdoomen dennisdoomen Jan 25, 2026

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Is that a suggestion? And why is that important? This code is only executed for a failing test and when the full details are requested.

@jnyrup jnyrup Jan 25, 2026

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It's a suggestion and not blocking for approving the PR.

@dennisdoomen dennisdoomen changed the title Fix FormatException when comparing strings containing braces Fix formatting exception when comparing strings containing braces Jan 26, 2026
@dennisdoomen
dennisdoomen merged commit 60b430a into main Jan 26, 2026
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@dennisdoomen
dennisdoomen deleted the copilot/fix-system-format-exception branch January 26, 2026 06:29
jnyrup pushed a commit to jnyrup/fluentassertions that referenced this pull request Feb 28, 2026
@jnyrup
Fix formatting exception when comparing strings containing braces (fl…
ad4470d 
…uentassertions#3151)
jnyrup pushed a commit to jnyrup/fluentassertions that referenced this pull request Mar 7, 2026
@jnyrup
Fix formatting exception when comparing strings containing braces (fl…
f542971 
…uentassertions#3151)
jnyrup pushed a commit that referenced this pull request Mar 8, 2026
@jnyrup
Fix formatting exception when comparing strings containing braces (#3151
16e5b2d 
)
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@dennisdoomen dennisdoomen dennisdoomen approved these changes
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Be() throws System.FormatException when expected value contains "{"

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