Improve error message when seeking to negative position in FileStream

src/libraries/System.Private.CoreLib/src/System/IO/Strategies/OSFileStreamStrategy.cs

@@ -158,8 +158,8 @@ public sealed override long Seek(long offset, SeekOrigin origin)
             }
             else
             {
-                // keep throwing the same exception we did when seek was causing actual offset change
-                FileStreamHelpers.ThrowInvalidArgument(_fileHandle);
+                // keep throwing the same exception type we did when seek was causing actual offset change
+                throw new IOException(SR.IO_SeekBeforeBegin);
             }
 
             // Prevent users from overwriting data in a file that was opened in append mode.

src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/FileStream/Seek.cs

@@ -7,6 +7,31 @@ namespace System.IO.Tests
 {
     public class FileStream_Seek : FileSystemTest
     {
+        [Theory]
+        [InlineData(0)]
+        [InlineData(10)]
+        public void SeekNegativePositionThrowsWithClearMessage(int bufferSize)
+        {
+            string fileName = GetTestFilePath();
+            using (FileStream fs = new FileStream(fileName, FileMode.Create, FileAccess.ReadWrite, FileShare.None, bufferSize))
+            {
+                fs.Write(TestBuffer, 0, TestBuffer.Length);
+
+                // Seek to negative position from Begin should throw IOException with clear message
+                IOException ex = Assert.Throws<IOException>(() => fs.Seek(-1, SeekOrigin.Begin));
+                Assert.Contains("before the beginning", ex.Message, StringComparison.OrdinalIgnoreCase);
+
+                // Seek to negative position from Current should throw IOException with clear message
+                fs.Position = 5;
+                ex = Assert.Throws<IOException>(() => fs.Seek(-10, SeekOrigin.Current));
+                Assert.Contains("before the beginning", ex.Message, StringComparison.OrdinalIgnoreCase);
+
+                // Seek to negative position from End should throw IOException with clear message
+                ex = Assert.Throws<IOException>(() => fs.Seek(-fs.Length - 1, SeekOrigin.End));
+                Assert.Contains("before the beginning", ex.Message, StringComparison.OrdinalIgnoreCase);
+            }
+        }
+
         [Theory]
         [InlineData(0)]
         [InlineData(10)]