Add failing test for issue #29494

src/Workspaces/CoreTestUtilities/Resources/ProjectFiles/CSharp/MallformedAdditionalFilePath.csproj

+<?xml version="1.0" encoding="utf-8"?>
+<Project ToolsVersion="15.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />
+  <PropertyGroup>
+    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
+    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
+    <ProjectGuid>{CE26EDB5-9F46-46D4-8EC0-9BC9293A8BE8}</ProjectGuid>
+    <OutputType>Library</OutputType>
+    <AppDesignerFolder>Properties</AppDesignerFolder>
+    <RootNamespace>MalformedProject</RootNamespace>
+    <AssemblyName>MalformedProject</AssemblyName>
+    <TargetFrameworkVersion>v4.0</TargetFrameworkVersion>
+    <FileAlignment>512</FileAlignment>
+    <Deterministic>true</Deterministic>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
+    <DebugSymbols>true</DebugSymbols>
+    <DebugType>full</DebugType>
+    <Optimize>false</Optimize>
+    <OutputPath>bin\Debug\</OutputPath>
+    <DefineConstants>DEBUG;TRACE</DefineConstants>
+    <ErrorReport>prompt</ErrorReport>
+    <WarningLevel>4</WarningLevel>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
+    <DebugType>pdbonly</DebugType>
+    <Optimize>true</Optimize>
+    <OutputPath>bin\Release\</OutputPath>
+    <DefineConstants>TRACE</DefineConstants>
+    <ErrorReport>prompt</ErrorReport>
+    <WarningLevel>4</WarningLevel>
+  </PropertyGroup>
+  <ItemGroup>
+    <Reference Include="System" />
+    <Reference Include="System.Core" />
+    <Reference Include="Microsoft.CSharp" />
+  </ItemGroup>
+  <ItemGroup>
+    <Compile Include="Class1.cs" />
+    <Compile Include="Properties\AssemblyInfo.cs" />
+  </ItemGroup>
+  <ItemGroup>
+    <AdditionalFiles Include="ValidAdditionalFile.txt" />
+    <AdditionalFiles Include="TEST::" />
+  </ItemGroup>
+  <ItemGroup />
+  <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
+</Project>
\ No newline at end of file

src/Workspaces/CoreTestUtilities/Resources/SourceFiles/Text/ValidAdditionalFile.txt

+
\ No newline at end of file

src/Workspaces/CoreTestUtilities/TestFiles/Resources.cs

@@ -126,6 +126,7 @@ public static class CSharp
                 public static string ExternAlias2 => GetText("ProjectFiles.CSharp.ExternAlias2.csproj");
                 public static string ForEmittedOutput => GetText("ProjectFiles.CSharp.ForEmittedOutput.csproj");
                 public static string MsbuildError => GetText("ProjectFiles.CSharp.MsbuildError.csproj");
+                public static string MallformedAdditionalFilePath => GetText("ProjectFiles.CSharp.MallformedAdditionalFilePath.csproj");
                 public static string NetCoreApp2_Project => GetText("NetCoreApp2.Project.csproj");
                 public static string NetCoreApp2AndLibrary_Project => GetText("NetCoreApp2AndLibrary.Project.csproj");
                 public static string NetCoreApp2AndLibrary_Library => GetText("NetCoreApp2AndLibrary.Library.csproj");
@@ -206,6 +207,11 @@ public static class FSharp
                 public static string NetCoreMultiTFM_ProjectReferenceToFSharp_FSharpLib_Library = GetText("NetCoreMultiTFM_ProjectReferenceToFSharp.fsharplib.Library.fs");
             }
 
+            public static class Text
+            {
+                public static string ValidAdditionalFile => GetText("SourceFiles.Text.ValidAdditionalFile.txt");
+            }
+
             public static class VisualBasic
             {
                 public static string Application => GetText("SourceFiles.VisualBasic.Application.myapp");

src/Workspaces/MSBuildTest/MSBuildWorkspaceTests.cs

@@ -3579,6 +3579,27 @@ public async Task TestOpenSolution_ProjectReferencesWithUnconventionalOutputPath
             }
         }
 
+        [ConditionalFact(typeof(VisualStudioMSBuildInstalled)), Trait(Traits.Feature, Traits.Features.MSBuildWorkspace)]
+        [WorkItem(29494, "https://github.com/dotnet/roslyn/issues/29494")]
+        public async Task TestOpenProjectAsync_MalformedAdditionalFilePath()
+        {
+            var files = GetSimpleCSharpSolutionFiles()
+                .WithFile(@"CSharpProject\CSharpProject.csproj", Resources.ProjectFiles.CSharp.MallformedAdditionalFilePath)
+                .WithFile(@"CSharpProject\ValidAdditionalFile.txt", Resources.SourceFiles.Text.ValidAdditionalFile);
+
+            CreateFiles(files);
+
+            var projectFilePath = GetSolutionFileName(@"CSharpProject\CSharpProject.csproj");
+
+            using (var workspace = CreateMSBuildWorkspace())
+            {
+                var project = await workspace.OpenProjectAsync(projectFilePath);
+
+                // Project should open without an exception being thrown.
+                Assert.NotNull(project);
+            }
+        }
+
         private class InMemoryAssemblyLoader : IAnalyzerAssemblyLoader
         {
             public void AddDependencyLocation(string fullPath)