Add Encoding.WithoutPreamble() extension method

PowerKit.Tests/EncodingExtensionsTests.cs

@@ -19,4 +19,48 @@ public void Utf8WithoutBom_Test()
         // Assert
         Encoding.UTF8.GetString(bytes).Should().Be(text);
     }
+
+    [Fact]
+    public void WithoutPreamble_Test()
+    {
+        // Arrange
+        var encoding = new UTF8Encoding(encoderShouldEmitUTF8Identifier: true);
+        encoding.GetPreamble().Should().NotBeEmpty();
+
+        // Act
+        var result = encoding.WithoutPreamble();
+
+        // Assert
+        result.GetPreamble().Should().BeEmpty();
+        result.GetString(result.GetBytes("hello")).Should().Be("hello");
+    }
+
+    [Fact]
+    public void WithoutPreamble_WithoutPreamble_Test()
+    {
+        // Arrange
+        var encoding = new UTF8Encoding(encoderShouldEmitUTF8Identifier: false);
+        encoding.GetPreamble().Should().BeEmpty();
+
+        // Act
+        var result = encoding.WithoutPreamble();
+
+        // Assert
+        result.Should().BeSameAs(encoding);
+    }
+
+    [Fact]
+    public void WithoutPreamble_RoundTrip_Test()
+    {
+        // Arrange
+        var text = "hello, world! 🌍";
+        var encoding = new UTF8Encoding(true).WithoutPreamble();
+
+        // Act
+        var bytes = encoding.GetBytes(text);
+        var decoded = encoding.GetString(bytes);
+
+        // Assert
+        decoded.Should().Be(text);
+    }
 }

PowerKit/Extensions/EncodingExtensions.cs

@@ -1,19 +1,60 @@
+using System;
 using System.Text;
 
 namespace PowerKit.Extensions;
 
+file sealed class NoPreambleEncoding(Encoding inner) : Encoding
+{
+    public override string BodyName => inner.BodyName;
+    public override string EncodingName => inner.EncodingName;
+    public override string HeaderName => inner.HeaderName;
+    public override string WebName => inner.WebName;
+    public override int CodePage => inner.CodePage;
+    public override bool IsBrowserDisplay => inner.IsBrowserDisplay;
+    public override bool IsBrowserSave => inner.IsBrowserSave;
+    public override bool IsMailNewsDisplay => inner.IsMailNewsDisplay;
+    public override bool IsMailNewsSave => inner.IsMailNewsSave;
+    public override bool IsSingleByte => inner.IsSingleByte;
+
+    public override byte[] GetPreamble() => [];
+
+    public override int GetByteCount(char[] chars, int index, int count) =>
+        inner.GetByteCount(chars, index, count);
+
+    public override int GetBytes(char[] chars, int charIndex, int charCount, byte[] bytes, int byteIndex) =>
+        inner.GetBytes(chars, charIndex, charCount, bytes, byteIndex);
+
+    public override int GetCharCount(byte[] bytes, int index, int count) =>
+        inner.GetCharCount(bytes, index, count);
+
+    public override int GetChars(byte[] bytes, int byteIndex, int byteCount, char[] chars, int charIndex) =>
+        inner.GetChars(bytes, byteIndex, byteCount, chars, charIndex);
+
+    public override int GetMaxByteCount(int charCount) => inner.GetMaxByteCount(charCount);
+
+    public override int GetMaxCharCount(int byteCount) => inner.GetMaxCharCount(byteCount);
+}
+
 file static class EncodingEx
 {
-    public static Encoding Utf8WithoutBom { get; } = new UTF8Encoding(false);
+    public static Encoding Utf8WithoutBom { get; } = Encoding.UTF8.WithoutPreamble();
 }
 
 internal static class EncodingExtensions
 {
-    extension(Encoding)
+    extension(Encoding encoding)
     {
         /// <summary>
         /// Gets an instance of the UTF-8 encoding that does not emit a byte order mark (BOM).
         /// </summary>
         public static Encoding Utf8WithoutBom => EncodingEx.Utf8WithoutBom;
+
+        /// <summary>
+        /// Creates a derived encoding that produces an empty preamble, regardless of the original encoding's preamble.
+        /// </summary>
+        public Encoding WithoutPreamble() =>
+            encoding.GetPreamble().Length > 0
+                ? new NoPreambleEncoding(encoding)
+                : encoding;
     }
 }