Fix #1175: Change IndexInputStream to throw NotSupportedException for writable members (#1231)

Changed Flush(), SetLength(), and Write() methods to throw NotSupportedException instead of IllegalStateException/InvalidCastException to be consistent with .NET conventions for read-only streams.

Added unit tests to verify the correct exception types are thrown.

Co-authored-by: PR Writer <prwriter@reveloexperts.com>

Author: sauravk58

src/Lucene.Net.Replicator/IndexInputInputStream.cs

@@ -38,7 +38,7 @@ public IndexInputStream(IndexInput input)
 
         public override void Flush()
         {
-            throw IllegalStateException.Create("Cannot flush a readonly stream."); // LUCENENET TODO: Change to NotSupportedException ?
+            throw new NotSupportedException("Cannot flush a readonly stream.");
         }
 
         public override long Seek(long offset, SeekOrigin origin)
@@ -60,7 +60,7 @@ public override long Seek(long offset, SeekOrigin origin)
 
         public override void SetLength(long value)
         {
-            throw IllegalStateException.Create("Cannot change length of a readonly stream."); // LUCENENET TODO: Change to NotSupportedException ?
+            throw new NotSupportedException("Cannot change length of a readonly stream.");
         }
 
         public override int Read(byte[] buffer, int offset, int count)
@@ -74,7 +74,7 @@ public override int Read(byte[] buffer, int offset, int count)
 
         public override void Write(byte[] buffer, int offset, int count)
         {
-            throw new InvalidCastException("Cannot write to a readonly stream."); // LUCENENET TODO: Change to NotSupportedException ?
+            throw new NotSupportedException("Cannot write to a readonly stream.");
         }
 
         public override bool CanRead => true;

src/Lucene.Net.Tests.Replicator/IndexInputStreamTest.cs

@@ -64,6 +64,51 @@ public void Read_RemainingIndexInputLargerThanReadCount_ReturnsExpectedSection([
             Assert.AreEqual(readBuffer, buffer.Skip((section - 1) * readBytes).Take(readBytes).ToArray());
         }
 
+        /// <summary>
+        /// Test for GitHub issue #1175: IndexInputStream should throw NotSupportedException for writable members
+        /// https://github.com/apache/lucenenet/issues/1175
+        /// </summary>
+        [Test]
+        [LuceneNetSpecific]
+        public void TestGitHubIssue1175_Flush_ThrowsNotSupportedException()
+        {
+            byte[] buffer = new byte[1.KiloBytes()];
+            Random.NextBytes(buffer);
+            using IndexInputStream stream = new IndexInputStream(new MockIndexInput(buffer));
+
+            Assert.Throws<NotSupportedException>(() => stream.Flush());
+        }
+
+        /// <summary>
+        /// Test for GitHub issue #1175: IndexInputStream should throw NotSupportedException for writable members
+        /// https://github.com/apache/lucenenet/issues/1175
+        /// </summary>
+        [Test]
+        [LuceneNetSpecific]
+        public void TestGitHubIssue1175_SetLength_ThrowsNotSupportedException()
+        {
+            byte[] buffer = new byte[1.KiloBytes()];
+            Random.NextBytes(buffer);
+            using IndexInputStream stream = new IndexInputStream(new MockIndexInput(buffer));
+
+            Assert.Throws<NotSupportedException>(() => stream.SetLength(100));
+        }
+
+        /// <summary>
+        /// Test for GitHub issue #1175: IndexInputStream should throw NotSupportedException for writable members
+        /// https://github.com/apache/lucenenet/issues/1175
+        /// </summary>
+        [Test]
+        [LuceneNetSpecific]
+        public void TestGitHubIssue1175_Write_ThrowsNotSupportedException()
+        {
+            byte[] buffer = new byte[1.KiloBytes()];
+            Random.NextBytes(buffer);
+            using IndexInputStream stream = new IndexInputStream(new MockIndexInput(buffer));
+
+            Assert.Throws<NotSupportedException>(() => stream.Write(new byte[10], 0, 10));
+        }
+
         /// <summary>
         /// Test for GitHub issue #1158: Integer overflow in IndexInputStream.Read()
         /// https://github.com/apache/lucenenet/issues/1158