Allow specifying EquivalencyOptions in string assertions

Src/FluentAssertions/Common/StringExtensions.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 using System.Linq;
 using System.Text.RegularExpressions;
 using FluentAssertions.Formatting;
@@ -9,15 +10,13 @@ internal static class StringExtensions
 {
     /// <summary>
     /// Finds the first index at which the <paramref name="value"/> does not match the <paramref name="expected"/>
-    /// string anymore, accounting for the specified <paramref name="stringComparison"/>.
+    /// string anymore, accounting for the specified <paramref name="comparer"/>.
     /// </summary>
-    public static int IndexOfFirstMismatch(this string value, string expected, StringComparison stringComparison)
+    public static int IndexOfFirstMismatch(this string value, string expected, IEqualityComparer<string> comparer)
     {
-        Func<char, char, bool> comparer = GetCharComparer(stringComparison);
-
         for (int index = 0; index < value.Length; index++)
         {
-            if (index >= expected.Length || !comparer(value[index], expected[index]))
+            if (index >= expected.Length || !comparer.Equals(value[index..(index + 1)], expected[index..(index + 1)]))
             {
                 return index;
             }
@@ -26,11 +25,6 @@ public static int IndexOfFirstMismatch(this string value, string expected, Strin
         return -1;
     }
 
-    private static Func<char, char, bool> GetCharComparer(StringComparison stringComparison) =>
-        stringComparison == StringComparison.Ordinal
-            ? (x, y) => x == y
-            : (x, y) => char.ToUpperInvariant(x) == char.ToUpperInvariant(y);
-
     /// <summary>
     /// Gets the quoted three characters at the specified index of a string, including the index itself.
     /// </summary>
@@ -129,23 +123,21 @@ public static string RemoveNewLines(this string @this)
     }
 
     /// <summary>
-    /// Counts the number of times a substring appears within a string by using the specified <see cref="StringComparison"/>.
+    /// Counts the number of times the <paramref name="substring"/> appears within a string by using the specified <paramref name="comparer"/>.
     /// </summary>
-    /// <param name="str">The string to search in.</param>
-    /// <param name="substring">The substring to search for.</param>
-    /// <param name="comparisonType">The <see cref="StringComparison"/> option to use for comparison.</param>
-    public static int CountSubstring(this string str, string substring, StringComparison comparisonType)
+    public static int CountSubstring(this string str, string substring, IEqualityComparer<string> comparer)
     {
         string actual = str ?? string.Empty;
         string search = substring ?? string.Empty;
 
         int count = 0;
-        int index = 0;
-
-        while ((index = actual.IndexOf(search, index, comparisonType)) >= 0)
+        int maxIndex = actual.Length - search.Length;
+        for (int index = 0; index <= maxIndex; index++)
         {
-            index += search.Length;
-            count++;
+            if (comparer.Equals(actual[index..(index + search.Length)], search))
+            {
+                count++;
+            }
         }
 
         return count;

Src/FluentAssertions/Equivalency/Execution/CollectionMemberOptionsDecorator.cs

@@ -73,5 +73,11 @@ public EqualityStrategy GetEqualityStrategy(Type type)
         return inner.GetEqualityStrategy(type);
     }
 
+    public bool IgnoreLeadingWhitespace => inner.IgnoreLeadingWhitespace;
+
+    public bool IgnoreTrailingWhitespace => inner.IgnoreTrailingWhitespace;
+
+    public bool IgnoreCase => inner.IgnoreCase;
+
     public ITraceWriter TraceWriter => inner.TraceWriter;
 }

Src/FluentAssertions/Equivalency/IEquivalencyOptions.cs

@@ -98,4 +98,19 @@ public interface IEquivalencyOptions
     /// Determines the right strategy for evaluating the equality of objects of this type.
     /// </summary>
     EqualityStrategy GetEqualityStrategy(Type type);
+
+    /// <summary>
+    /// Gets a value indicating whether leading whitespace is ignored when comparing <see langword="string" />s.
+    /// </summary>
+    bool IgnoreLeadingWhitespace { get; }
+
+    /// <summary>
+    /// Gets a value indicating whether trailing whitespace is ignored when comparing <see langword="string" />s.
+    /// </summary>
+    bool IgnoreTrailingWhitespace { get; }
+
+    /// <summary>
+    /// Gets a value indicating whether a case-insensitive comparer is used when comparing <see langword="string" />s.
+    /// </summary>
+    bool IgnoreCase { get; }
 }

Src/FluentAssertions/Equivalency/SelfReferenceEquivalencyOptions.cs

@@ -56,6 +56,8 @@ public abstract class SelfReferenceEquivalencyOptions<TSelf> : IEquivalencyOptio
     private bool ignoreNonBrowsableOnSubject;
     private bool excludeNonBrowsableOnExpectation;
 
+    private IEqualityComparer<string> stringComparer;
+
     #endregion
 
     private protected SelfReferenceEquivalencyOptions()
@@ -86,6 +88,9 @@ protected SelfReferenceEquivalencyOptions(IEquivalencyOptions defaults)
         includedFields = defaults.IncludedFields;
         ignoreNonBrowsableOnSubject = defaults.IgnoreNonBrowsableOnSubject;
         excludeNonBrowsableOnExpectation = defaults.ExcludeNonBrowsableOnExpectation;
+        IgnoreLeadingWhitespace = defaults.IgnoreLeadingWhitespace;
+        IgnoreTrailingWhitespace = defaults.IgnoreTrailingWhitespace;
+        IgnoreCase = defaults.IgnoreCase;
 
         ConversionSelector = defaults.ConversionSelector.Clone();
 
@@ -180,6 +185,12 @@ IEnumerable<IMemberSelectionRule> IEquivalencyOptions.SelectionRules
     EqualityStrategy IEquivalencyOptions.GetEqualityStrategy(Type type)
         => equalityStrategyProvider.GetEqualityStrategy(type);
 
+    public bool IgnoreLeadingWhitespace { get; private set; }
+
+    public bool IgnoreTrailingWhitespace { get; private set; }
+
+    public bool IgnoreCase { get; private set; }
+
     public ITraceWriter TraceWriter { get; private set; }
 
     /// <summary>
@@ -497,6 +508,18 @@ public TSelf Using<T>(IEqualityComparer<T> comparer)
         return (TSelf)this;
     }
 
+    /// <summary>
+    /// Ensures the equivalency comparison will use the specified implementation of <see cref="IEqualityComparer{String}"/>
+    /// any time when a property is a <see langword="string" />.
+    /// </summary>
+    public TSelf Using(IEqualityComparer<string> comparer)
+    {
+        userEquivalencySteps.Insert(0, new EqualityComparerEquivalencyStep<string>(comparer));
+        stringComparer = comparer;
+
+        return (TSelf)this;
+    }
+
     /// <summary>
     /// Causes all collections to be compared in the order in which the items appear in the expectation.
     /// </summary>
@@ -679,6 +702,44 @@ public TSelf WithoutAutoConversionFor(Expression<Func<IObjectInfo, bool>> predic
         return (TSelf)this;
     }
 
+    /// <summary>
+    /// Instructs the comparison to ignore leading whitespace when comparing <see langword="string" />s.
+    /// </summary>
+    /// <remarks>
+    /// Note: This affects the index of first mismatch, as the removed whitespace is also ignored for the index calculation!
+    /// </remarks>
+    public TSelf IgnoringLeadingWhitespace()
+    {
+        IgnoreLeadingWhitespace = true;
+        return (TSelf)this;
+    }
+
+    /// <summary>
+    /// Instructs the comparison to ignore trailing whitespace when comparing <see langword="string" />s.
+    /// </summary>
+    public TSelf IgnoringTrailingWhitespace()
+    {
+        IgnoreTrailingWhitespace = true;
+        return (TSelf)this;
+    }
+
+    /// <summary>
+    /// Instructs the comparison to compare <see langword="string" />s case-insensitive.
+    /// </summary>
+    public TSelf IgnoringCase()
+    {
+        IgnoreCase = true;
+        return (TSelf)this;
+    }
+
+    /// <summary>
+    /// Returns the comparer for strings, which is either an explicitly specified comparer via <see cref="Using{T}(System.Collections.Generic.IEqualityComparer{T})"/> or an ordinal comparer depending on <see cref="IgnoringCase()" />.
+    /// </summary>
+    internal IEqualityComparer<string> GetStringComparerOrDefault()
+    {
+        return stringComparer ?? (IgnoreCase ? StringComparer.OrdinalIgnoreCase : StringComparer.Ordinal);
+    }
+
     /// <summary>
     /// Returns a string that represents the current object.
     /// </summary>

Src/FluentAssertions/Equivalency/Steps/StringEqualityEquivalencyStep.cs

@@ -28,12 +28,40 @@ public class StringEqualityEquivalencyStep : IEquivalencyStep
             string expectation = (string)comparands.Expectation;
 
             subject.Should()
-                .Be(expectation, context.Reason.FormattedMessage, context.Reason.Arguments);
+                .Be(expectation, CreateOptions(context.Options),
+                    context.Reason.FormattedMessage, context.Reason.Arguments);
         }
 
         return EquivalencyResult.AssertionCompleted;
     }
 
+    private static Func<EquivalencyOptions<string>, EquivalencyOptions<string>>
+        CreateOptions(IEquivalencyOptions existingOptions) =>
+        o =>
+        {
+            if (existingOptions is EquivalencyOptions<string> equivalencyOptions)
+            {
+                return equivalencyOptions;
+            }
+
+            if (existingOptions.IgnoreLeadingWhitespace)
+            {
+                o.IgnoringLeadingWhitespace();
+            }
+
+            if (existingOptions.IgnoreTrailingWhitespace)
+            {
+                o.IgnoringTrailingWhitespace();
+            }
+
+            if (existingOptions.IgnoreCase)
+            {
+                o.IgnoringCase();
+            }
+
+            return o;
+        };
+
     private static bool ValidateAgainstNulls(Comparands comparands, INode currentNode)
     {
         object expected = comparands.Expectation;