Write UTF-8 BOM for solution files.

Currently the solution file written out by the `sln` command uses a
UTF-8 encoding without a BOM.  This causes problems when the solution
file contains non-ASCII code points because Visual Studio and MSBuild
will not use a UTF-8 encoding when reading the solution file if the
BOM is omitted.

This commit causes the BOM to always be written when writing the
solution files.

Fixes #8184.

src/Microsoft.DotNet.Cli.Sln.Internal/SlnFile.cs

@@ -33,6 +33,7 @@ using System.IO;
 using System.Collections;
 using System.Globalization;
 using System.Reflection;
+using System.Text;
 using Microsoft.DotNet.Cli.Sln.Internal.FileManipulation;
 using Microsoft.DotNet.Tools.Common;
 
@@ -211,7 +212,7 @@ namespace Microsoft.DotNet.Cli.Sln.Internal
             }
             var sw = new StringWriter();
             Write(sw);
-            File.WriteAllText(FullPath, sw.ToString());
+            File.WriteAllText(FullPath, sw.ToString(), Encoding.UTF8);
         }
 
         private void Write(TextWriter writer)

test/dotnet-sln-add.Tests/GivenDotnetSlnAdd.cs

@@ -9,6 +9,7 @@ using Microsoft.DotNet.Tools.Test.Utilities;
 using System;
 using System.IO;
 using System.Linq;
+using System.Text;
 using Xunit;
 using Xunit.Abstractions;
 
@@ -554,6 +555,33 @@ EndGlobal
             cmd.StdErr.Should().BeEmpty();
         }
 
+        [Fact]
+        public void WhenProjectIsAddedSolutionHasUTF8BOM()
+        {
+            var projectDirectory = TestAssets
+                .Get("TestAppWithEmptySln")
+                .CreateInstance()
+                .WithSourceFiles()
+                .Root
+                .FullName;
+
+            var projectToAdd = "Lib/Lib.csproj";
+            var projectPath = Path.Combine("Lib", "Lib.csproj");
+            var cmd = new DotnetCommand()
+                .WithWorkingDirectory(projectDirectory)
+                .ExecuteWithCapturedOutput($"sln App.sln add {projectToAdd}");
+            cmd.Should().Pass();
+
+            var preamble = Encoding.UTF8.GetPreamble();
+            preamble.Length.Should().Be(3);
+            using (var stream = new FileStream(Path.Combine(projectDirectory, "App.sln"), FileMode.Open))
+            {
+                var bytes = new byte[preamble.Length];
+                stream.Read(bytes, 0, bytes.Length);
+                bytes.Should().BeEquivalentTo(preamble);
+            }
+        }
+
         [Theory]
         [InlineData("TestAppWithSlnAndCsprojFiles")]
         [InlineData("TestAppWithSlnAndCsprojProjectGuidFiles")]

test/dotnet-sln-remove.Tests/GivenDotnetSlnRemove.cs

@@ -8,6 +8,7 @@ using Microsoft.DotNet.Tools.Test.Utilities;
 using System;
 using System.IO;
 using System.Linq;
+using System.Text;
 using Xunit;
 
 namespace Microsoft.DotNet.Cli.Sln.Remove.Tests
@@ -590,6 +591,32 @@ EndGlobal
                 .Count().Should().Be(1, $"App {reasonString}");
         }
 
+        [Fact]
+        public void WhenProjectIsRemovedSolutionHasUTF8BOM()
+        {
+            var projectDirectory = TestAssets
+                .Get("TestAppWithSlnAndCsprojToRemove")
+                .CreateInstance()
+                .WithSourceFiles()
+                .Root
+                .FullName;
+
+            var projectToRemove = Path.Combine("Lib", "Lib.csproj");
+            var cmd = new DotnetCommand()
+                .WithWorkingDirectory(projectDirectory)
+                .ExecuteWithCapturedOutput($"sln App.sln remove {projectToRemove}");
+            cmd.Should().Pass();
+
+            var preamble = Encoding.UTF8.GetPreamble();
+            preamble.Length.Should().Be(3);
+            using (var stream = new FileStream(Path.Combine(projectDirectory, "App.sln"), FileMode.Open))
+            {
+                var bytes = new byte[preamble.Length];
+                stream.Read(bytes, 0, bytes.Length);
+                bytes.Should().BeEquivalentTo(preamble);
+            }
+        }
+
         [Fact]
         public void WhenFinalReferenceIsRemovedEmptySectionsAreRemoved()
         {