Report mismatch segment for strings of unequal lengths

Src/FluentAssertions/Primitives/StringEqualityValidator.cs

@@ -27,14 +27,51 @@ protected override bool ValidateAgainstSuperfluousWhitespace()
 
         protected override bool ValidateAgainstLengthDifferences()
         {
+            // Logic is a little bit convoluted because I want to avoid calculation
+            // of mismatch segment in case of equalLength == true for performance reason.
+            // If lazy version of FailWith would be introduced, calculation of mismatch
+            // segment can be moved directly to FailWith's argument
+            bool equalLength = subject.Length == expected.Length;
+
+            string mismatchSegment = GetMismatchSegmentForStringsOfDifferentLengths(equalLength);
+
             return assertion
-                .ForCondition(subject.Length == expected.Length)
+                .ForCondition(equalLength)
                 .FailWith(
-                    ExpectationDescription + "{0} with a length of {1}{reason}, but {2} has a length of {3}.",
+                    ExpectationDescription + "{0} with a length of {1}{reason}, but {2} has a length of {3}, differs near " + mismatchSegment +  ".",
                     expected, expected.Length, subject, subject.Length)
                 .SourceSucceeded;
         }
 
+        private string GetMismatchSegmentForStringsOfDifferentLengths(bool equalLength)
+        {
+            if (equalLength)
+            {
+                return "";
+            }
+
+            int indexOfMismatch = subject.IndexOfFirstMismatch(expected, comparisonMode);
+
+            // If there is no difference it means that subject and expected have common prefix
+            // and the first difference is after just that prefix.
+            if (indexOfMismatch == -1)
+            {
+                if (subject.Length < expected.Length)
+                {
+                    // If subject is shorter, we point at its last character
+                    indexOfMismatch = Math.Max(0, subject.Length - 1);
+                }
+                else
+                {
+                    // If subject is longer we point at first character after expected
+                    indexOfMismatch = Math.Max(0, expected.Length);
+                }
+            }
+
+            string mismatchSegment = subject.IndexedSegmentAt(indexOfMismatch);
+            return mismatchSegment;
+        }
+
         protected override void ValidateAgainstMismatch()
         {
             int indexOfMismatch = subject.IndexOfFirstMismatch(expected, comparisonMode);

Tests/Shared.Specs/BasicEquivalencySpecs.cs

@@ -644,7 +644,7 @@ public void
             // Assert
             //-----------------------------------------------------------------------------------------------------------
             act.Should().Throw<XunitException>().WithMessage(
-                @"Expected member Member1 to be*""different"" with a length of 9, but*"""" has a length of 0.*");
+                @"Expected member Member1 to be*""different"" with a length of 9, but*"""" has a length of 0*");
         }
 
         [Fact]

Tests/Shared.Specs/StringAssertionSpecs.cs

@@ -66,7 +66,7 @@ public void When_the_expected_string_is_shorter_than_the_actual_string_it_should
             // Assert
             //-----------------------------------------------------------------------------------------------------------
             act.Should().Throw<XunitException>().WithMessage(
-                "Expected string to be \"AB\" with a length of 2, but \"ABC\" has a length of 3.");
+                "Expected string to be \"AB\" with a length of 2, but \"ABC\" has a length of 3, differs near \"C\" (index 2).");
         }
 
         [Fact]
@@ -81,7 +81,37 @@ public void When_the_expected_string_is_longer_than_the_actual_string_it_should_
             // Assert
             //-----------------------------------------------------------------------------------------------------------
             act.Should().Throw<XunitException>().WithMessage(
-                "Expected string to be \"ABC\" with a length of 3, but \"AB\" has a length of 2.");
+                "Expected string to be \"ABC\" with a length of 3, but \"AB\" has a length of 2, differs near \"B\" (index 1).");
+        }
+
+        [Fact]
+        public void When_the_expected_string_is_empty_it_should_throw()
+        {
+            //-----------------------------------------------------------------------------------------------------------
+            // Act
+            //-----------------------------------------------------------------------------------------------------------
+            Action act = () => "ABC".Should().Be("");
+
+            //-----------------------------------------------------------------------------------------------------------
+            // Assert
+            //-----------------------------------------------------------------------------------------------------------
+            act.Should().Throw<XunitException>().WithMessage(
+                "Expected string to be \"\" with a length of 0, but \"ABC\" has a length of 3, differs near \"ABC\" (index 0).");
+        }
+
+        [Fact]
+        public void When_the_subject_string_is_empty_it_should_throw()
+        {
+            //-----------------------------------------------------------------------------------------------------------
+            // Act
+            //-----------------------------------------------------------------------------------------------------------
+            Action act = () => "".Should().Be("ABC");
+
+            //-----------------------------------------------------------------------------------------------------------
+            // Assert
+            //-----------------------------------------------------------------------------------------------------------
+            act.Should().Throw<XunitException>().WithMessage(
+                "Expected string to be \"ABC\" with a length of 3, but \"\" has a length of 0, differs near \"\" (index 0).");
         }
 
         [Fact]
@@ -1904,7 +1934,7 @@ public void When_non_empty_string_is_expected_to_be_equivalent_to_empty_it_shoul
             // Assert
             //-----------------------------------------------------------------------------------------------------------
             act.Should().Throw<XunitException>().WithMessage(
-                "Expected string to be equivalent to \"\" with a length of 0, but \"ABC\" has a length of 3.");
+                "Expected string to be equivalent to \"\" with a length of 0, but \"ABC\" has a length of 3, differs near \"ABC\" (index 0).");
         }
 
         [Fact]

docs/_pages/documentation.md

@@ -1290,7 +1290,7 @@ The above will batch the two failures, and throw an exception at the point of di
 E.g. Exception thrown at point of dispose contains:
 
     Expected value to be 10, but found 5.
-    Expected string to be "Expected" with a length of 8, but "Actual" has a length of 6.
+    Expected string to be "Expected" with a length of 8, but "Actual" has a length of 6, differs near "Act" (index 0).
 
         at........