Fix IndexOutOfRangeException in reader metadata after exhausting result. Fixes #332

DataTable.Load (and similar consumers) read all rows and then call
NextResult() to probe for more result sets. InitNextReader() disposed the
vector readers and set vectorReaders=[] at the top of the method, before
checking whether another result set existed. When none did, it returned
false with vectorReaders emptied but fieldCount still holding the previous
result's column count. A subsequent GetSchemaTable()/GetName()/GetFieldType()
then indexed past the emptied array and threw IndexOutOfRangeException.

This regressed in 1.5.0 with the cross-chunk reader-reuse refactor; in 1.4.4
the readers were only replaced when a new result set was found, so an
exhausted reader kept its column metadata.

Move the dispose + clear inside the QueryResult branch so they run only when
switching to a new result set. An exhausted reader now retains its column
metadata, restoring 1.4.4 behavior.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Giorgi

DuckDB.NET.Data/DuckDBDataReader.cs

@@ -38,18 +38,18 @@ internal DuckDBDataReader(DuckDBCommand command, IEnumerable<DuckDBResult> query
 
     private bool InitNextReader()
     {
-        foreach (var reader in vectorReaders)
-        {
-            reader?.Dispose();
-        }
-
-        vectorReaders = [];
-
         while (resultEnumerator.MoveNext())
         {
             var result = resultEnumerator.Current;
             if (NativeMethods.Query.DuckDBResultReturnType(result) == DuckDBResultType.QueryResult)
             {
+                foreach (var reader in vectorReaders)
+                {
+                    reader?.Dispose();
+                }
+
+                vectorReaders = [];
+
                 currentChunkIndex = 0;
                 currentResult = result;
 

DuckDB.NET.Test/DuckDBDataReaderTests.cs

@@ -353,6 +353,35 @@ public void ReadDecimalSchema()
         schemaTable.Rows[0]["NumericPrecision"].Should().Be(10);
     }
 
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public void Repro332_SchemaTableAfterNextResult(bool streaming)
+    {
+        Command.CommandText = "SELECT 1 AS a, 2 AS b";
+        Command.UseStreamingMode = streaming;
+        using var reader = Command.ExecuteReader();
+
+        while (reader.Read()) { }
+        reader.NextResult().Should().BeFalse(); // exhausts result set, like DataTable.Load does
+
+        var schemaTable = reader.GetSchemaTable();
+        schemaTable.Rows.Count.Should().Be(2);
+    }
+
+    [Fact]
+    public void Repro332_DataTableLoad()
+    {
+        Command.CommandText = "SELECT 1 AS a, 2 AS b";
+        using var reader = Command.ExecuteReader();
+
+        var table = new DataTable();
+        table.Load(reader); // calls NextResult() internally after reading rows
+
+        var schemaTable = reader.GetSchemaTable();
+        schemaTable.Rows.Count.Should().Be(2);
+    }
+
     [Fact]
     public void ReadDecimalSchemaWithoutTableRow()
     {