Make task environment path absolutization not throw.

src/Build.UnitTests/BackEnd/TaskEnvironment_Tests.cs

@@ -358,5 +358,30 @@ public void TaskEnvironment_MultithreadedEnvironment_ShouldBeIsolatedFromSystem(
                 Environment.SetEnvironmentVariable(testVarName, null);
             }
         }
+
+        [Theory]
+        [MemberData(nameof(EnvironmentTypes))]
+        public void TaskEnvironment_GetAbsolutePath_WithInvalidPathChars_ShouldNotThrow(string environmentType)
+        {
+            // Construct a path containing an invalid path character
+            char invalidChar = Path.GetInvalidPathChars().FirstOrDefault();
+            string invalidPath = "invalid" + invalidChar + "path";
+
+            var taskEnvironment = CreateTaskEnvironment(environmentType);
+
+            try
+            {
+                // Should not throw on invalid path characters
+                var absolutePath = taskEnvironment.GetAbsolutePath(invalidPath);
+
+                // The result should contain the invalid path combined with the base directory
+                absolutePath.Value.ShouldNotBeNullOrEmpty();
+                absolutePath.Value.ShouldContain(invalidPath);
+            }
+            finally
+            {
+                DisposeTaskEnvironment(taskEnvironment);
+            }
+        }
     }
 }

src/Build/BackEnd/TaskExecutionHost/MultiProcessTaskEnvironmentDriver.cs

@@ -4,7 +4,11 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
+#if NETFRAMEWORK
+using Microsoft.IO;
+#else
 using System.IO;
+#endif
 using Microsoft.Build.Framework;
 using Microsoft.Build.Internal;
 
@@ -41,7 +45,10 @@ public AbsolutePath ProjectDirectory
         /// <inheritdoc/>
         public AbsolutePath GetAbsolutePath(string path)
         {
-            return new AbsolutePath(Path.GetFullPath(path), ignoreRootedCheck: true);
+            // This function should not throw when path has illegal characters.
+            // For .NET Framework, Microsoft.IO.Path.Combine should be used instead of System.IO.Path.Combine to achieve it.
+            // For .NET Core, System.IO.Path.Combine already does not throw in this case.
+            return new AbsolutePath(Path.Combine(NativeMethodsShared.GetCurrentDirectory(), path), ignoreRootedCheck: true);
         }
 
         /// <inheritdoc/>

src/Build/BackEnd/TaskExecutionHost/MultiThreadedTaskEnvironmentDriver.cs

@@ -63,7 +63,7 @@ public AbsolutePath ProjectDirectory
             {
                 _currentDirectory = value;
                 // Keep the thread-static in sync for use by Expander and Modifiers during property/item expansion.
-                // This allows Path.GetFullPath and %(FullPath) to resolve relative paths correctly in multithreaded mode.
+                // This allows Path.GetFullPath and %(FullPath) functions used in project files to resolve relative paths correctly in multithreaded mode.
                 FileUtilities.CurrentThreadWorkingDirectory = value.Value;
             }
         }

src/Framework/TaskEnvironment.cs

@@ -33,7 +33,7 @@ public AbsolutePath ProjectDirectory
 
         /// <summary>
         /// Converts a relative or absolute path string to an absolute path.
-        /// This function resolves paths relative to ProjectDirectory.
+        /// This function resolves paths relative to <see cref="ProjectDirectory"/>.
         /// </summary>
         /// <param name="path">The path to convert.</param>
         /// <returns>An absolute path representation.</returns>
@@ -77,4 +77,4 @@ public AbsolutePath ProjectDirectory
         /// </summary>
         internal void Dispose() => _driver.Dispose();
     }
-}
+}