dotnet · huulinhnguyen-dev · Dec 10, 2025 · Dec 17, 2025
diff --git a/src/Build.UnitTests/BackEnd/AssemblyLoadContextTestTasks.cs b/src/Build.UnitTests/BackEnd/AssemblyLoadContextTestTasks.cs
@@ -3,11 +3,69 @@
 
 using Microsoft.Build.Framework;
 using Microsoft.Build.Utilities;
+using System;
+using System.Reflection;
 
 #nullable disable
 
 namespace AssemblyLoadContextTest
 {
+    /// <summary>
+    /// Task that validates assembly version roll-forward behavior.
+    /// Tests that MSBuildLoadContext accepts newer assembly versions when older versions are requested.
+    /// </summary>
+    public class ValidateAssemblyVersionRollForward : Task
+    {
+        /// <summary>
+        /// The name of the assembly to check (e.g., "System.Collections.Immutable")
+        /// </summary>
+        [Required]
+        public string AssemblyName { get; set; }
+
+        /// <summary>
+        /// The minimum expected version (e.g., "1.0.0.0")
+        /// </summary>
+        [Required]
+        public string MinimumVersion { get; set; }
+
+        public override bool Execute()
+        {
+            try
+            {
+                // Try to load the assembly by name with minimum version
+                var minimumVersion = Version.Parse(MinimumVersion);
+                var assemblyName = new AssemblyName(AssemblyName)
+                {
+                    Version = minimumVersion
+                };
+
+                // This will trigger MSBuildLoadContext.Load which should accept newer versions
+                var assembly = Assembly.Load(assemblyName);
+                var loadedVersion = assembly.GetName().Version;
+
+                Log.LogMessage(MessageImportance.High,
+                    $"Requested {AssemblyName} version {minimumVersion}, loaded version {loadedVersion}");
+
+                // Verify that we got a version >= minimum
+                if (loadedVersion < minimumVersion)
+                {
+                    Log.LogError(
+                        $"Assembly version roll-forward failed: requested {minimumVersion}, but loaded {loadedVersion} which is older");
+                    return false;
+                }
+
+                Log.LogMessage(MessageImportance.High,
+                    $"Assembly version roll-forward succeeded: loaded version {loadedVersion} >= requested {minimumVersion}");
+                return true;
+            }
+            catch (Exception ex)
+            {
+                Log.LogErrorFromException(ex, showStackTrace: true);
+                return false;
+            }
+        }
+    }
+
     public class RegisterObject : Task
     {
         internal const string CacheKey = "RegressionForMSBuild#5080";

diff --git a/src/Build.UnitTests/BackEnd/TaskBuilder_Tests.cs b/src/Build.UnitTests/BackEnd/TaskBuilder_Tests.cs
@@ -662,6 +662,33 @@ public void SameAssemblyFromDifferentRelativePathsSharesAssemblyLoadContext()
             logger.AssertLogDoesntContain("MSB4018");
         }
 
+#if FEATURE_ASSEMBLYLOADCONTEXT
+        /// <summary>
+        /// Regression test for https://github.com/dotnet/msbuild/issues/12370
+        /// Verifies that MSBuildLoadContext accepts newer assembly versions when older versions are requested (version roll-forward).
+        /// </summary>
+        [Fact]
+        public void MSBuildLoadContext_AcceptsNewerAssemblyVersions()
+        {
+            string realTaskPath = Assembly.GetExecutingAssembly().Location;
+
+            // Use System.Collections.Immutable as test assembly - it's available in modern .NET runtime
+            // Request an older version (1.0.0.0) which should roll forward to whatever version is available
+            string projectContents = @"<Project ToolsVersion=`msbuilddefaulttoolsversion` xmlns=`msbuildnamespace`>
+  <UsingTask TaskName=`ValidateAssemblyVersionRollForward` AssemblyFile=`" + realTaskPath + @"` />
+
+  <Target Name=`Build`>
+    <ValidateAssemblyVersionRollForward AssemblyName=`System.Collections.Immutable` MinimumVersion=`1.0.0.0` />
+  </Target>
+</Project>";
+
+            MockLogger logger = ObjectModelHelpers.BuildProjectExpectSuccess(projectContents, _testOutput);
+
+            // Verify that the task logged success message
+            logger.AssertLogContains("Assembly version roll-forward succeeded");
+        }
+#endif
+
 
 #if FEATURE_CODETASKFACTORY
         /// <summary>

diff --git a/src/Shared/MSBuildLoadContext.cs b/src/Shared/MSBuildLoadContext.cs
@@ -83,7 +83,7 @@ public MSBuildLoadContext(string assemblyPath)
                 }
 
                 AssemblyName candidateAssemblyName = AssemblyLoadContext.GetAssemblyName(candidatePath);
-                if (candidateAssemblyName.Version != assemblyName.Version)
+                if (candidateAssemblyName.Version < assemblyName.Version)
                 {
                     continue;
                 }