Displaying a diff when the assert fails

src/Snapshooter.MSTest/MSTestAssert.cs

@@ -1,3 +1,4 @@
+using Microsoft.VisualStudio.TestTools.UnitTesting;
 using Snapshooter.Core;
 using MsTest = Microsoft.VisualStudio.TestTools.UnitTesting;
 
@@ -16,7 +17,16 @@ public class MSTestAssert : IAssert
         /// <param name="actualSnapshot">The actual snapshot.</param>
         public void Assert(string expectedSnapshot, string actualSnapshot)
         {
-            MsTest.Assert.AreEqual(expectedSnapshot, actualSnapshot);
+            try
+            {
+                MsTest.Assert.AreEqual(expectedSnapshot, actualSnapshot);
+            }
+            catch (AssertFailedException ex)
+            {
+                string displayableDiff = DiffGenerator.GenerateDiff(expectedSnapshot, actualSnapshot);
+
+                throw new AssertFailedException($"Assert.AreEqual failed. {displayableDiff}", ex);
+            }
         }
     }
 }

src/Snapshooter.NUnit/NUnitAssert.cs

@@ -1,4 +1,3 @@
-using System.Diagnostics;
 using NUnit.Framework;
 using Snapshooter.Core;
 using NAssert = NUnit.Framework.Assert;
@@ -18,7 +17,15 @@ public class NUnitAssert : IAssert
         /// <param name="actualSnapshot">The actual snapshot.</param>
         public void Assert(string expectedSnapshot, string actualSnapshot)
         {
-            NAssert.That(actualSnapshot, Is.EqualTo(expectedSnapshot));
+            try
+            {
+                NAssert.That(actualSnapshot, Is.EqualTo(expectedSnapshot));
+            } catch (AssertionException ex)
+            {
+                string displayableDiff = DiffGenerator.GenerateDiff(expectedSnapshot, actualSnapshot);
+
+                throw new AssertionException($"Assert.That failed. {displayableDiff}", ex);
+            }
         }
     }
 }

src/Snapshooter.Xunit/XunitAssert.cs

@@ -1,4 +1,5 @@
-using Snapshooter.Core;
+using Snapshooter.Core;
+using Xunit.Sdk;
 using x = Xunit;
 
 namespace Snapshooter.Xunit
@@ -16,7 +17,15 @@ public class XunitAssert : IAssert
         /// <param name="actualSnapshot">The actual snapshot.</param>
         public void Assert(string expectedSnapshot, string actualSnapshot)
         {
-            x.Assert.Equal(expectedSnapshot, actualSnapshot);
+            try
+            {
+                x.Assert.Equal(expectedSnapshot, actualSnapshot);
+            }
+            catch (EqualException ex)
+            {
+                string displayableDiff = DiffGenerator.GenerateDiff(expectedSnapshot, actualSnapshot);
+                throw new EqualException($"Assert.Equal failed. {displayableDiff}", ex);
+            }
         }
     }
 }

src/Snapshooter/DiffGenerator.cs

@@ -0,0 +1,33 @@
+using System;
+using System.Linq;
+using DiffPlex.DiffBuilder;
+using DiffPlex.DiffBuilder.Model;
+
+namespace Snapshooter
+{
+    /// <summary>
+    /// Class for generating a diff between two strings.
+    /// </summary>
+    public static class DiffGenerator
+    {
+        /// <summary>
+        /// Generate a displayable diff between two strings.
+        /// </summary>
+        /// <param name="expectedSnapshot">The expected snapshot used in the assert.</param>
+        /// <param name="actualSnapshot">The actual snapshot used in the assert.</param>
+        /// <returns>A new string showing the added/removed lines between the two snapshots.</returns>
+        public static string GenerateDiff(string expectedSnapshot, string actualSnapshot)
+        {
+            DiffPaneModel diff = InlineDiffBuilder.Diff(expectedSnapshot, actualSnapshot);
+            string displayableDiff = string.Join(
+                Environment.NewLine,
+                diff.Lines.Select(line => line.Type switch
+                    {
+                        ChangeType.Inserted => $"+ {line.Text}",
+                        ChangeType.Deleted => $"- {line.Text}",
+                        _ => line.Text
+                    }));
+            return displayableDiff;
+        }
+    }
+}

src/Snapshooter/Snapshooter.csproj

@@ -14,6 +14,7 @@
 
   <ItemGroup>
     <PackageReference Include="Newtonsoft.Json" Version="13.0.3" />
+    <PackageReference Include="DiffPlex" Version="1.7.1" />
   </ItemGroup>
 
 </Project>