avoid duplicate UTF8Preamble (#542)

src/Directory.Build.props

@@ -2,7 +2,7 @@
 <Project>
   <PropertyGroup>
     <NoWarn>CS1591;CS0649;xUnit1026;xUnit1013;msb3277;CS0436</NoWarn>
-    <Version>17.1.2</Version>
+    <Version>17.1.3</Version>
     <ImplicitUsings>enable</ImplicitUsings>
     <LangVersion>10</LangVersion>
     <SuppressTfmSupportBuildWarnings>true</SuppressTfmSupportBuildWarnings>

src/Verify.Tests/Tests.StringWithUtf8Bom.verified.txt

@@ -0,0 +1 @@
+a

src/Verify.Tests/Tests.cs

@@ -492,6 +492,16 @@ public Task ShouldThrowForExtensionOnSerialization()
         return Verifier.ThrowsTask(() => Verify(element, settings))
             .IgnoreStackTrace();
     }
+#if NET6_0
+    [Fact]
+    public async Task StringWithUtf8Bom()
+    {
+        var utf8 = Encoding.UTF8;
+        var preamble = utf8.GetString(utf8.GetPreamble());
+        await Verify($"{preamble}a").AutoVerify();
+        await Verify("a").DisableRequireUniquePrefix();
+    }
+#endif
 
     [Fact]
     public Task StringExtension()

src/Verify/IoHelpers.cs

@@ -2,6 +2,8 @@
 {
     static readonly UTF8Encoding Utf8 = new(true, true);
 
+    static readonly string UTF8Preamble = Encoding.UTF8.GetString(Encoding.UTF8.GetPreamble());
+
     public static void DeleteIfEmpty(string path)
     {
         var info = new FileInfo(path);
@@ -43,9 +45,19 @@ static async Task<string> ReadStringWithFixedLines(this Stream stream)
         return builder.ToString();
     }
 
+    static string TrimPreamble(string text)
+    {
+        if (text.StartsWith(UTF8Preamble))
+        {
+            return text.Substring(UTF8Preamble.Length);
+        }
+
+        return text;
+    }
+
 #if NETSTANDARD2_1 || NET5_0_OR_GREATER
     public static Task WriteText(string path, string text) =>
-        File.WriteAllTextAsync(path, text, Utf8);
+        File.WriteAllTextAsync(path, TrimPreamble(text), Utf8);
 
     public static async Task<string> ReadStringWithFixedLines(string path)
     {
@@ -68,7 +80,7 @@ public static async Task WriteStream(string path, Stream stream)
 #else
     public static async Task WriteText(string path, string text)
     {
-        var encodedText = Utf8.GetBytes(text);
+        var encodedText = Utf8.GetBytes(TrimPreamble(text));
 
         using var stream = OpenWrite(path);
         await stream.WriteAsync(encodedText, 0, encodedText.Length);