fix for issue bordoley#40 (helpful exception on invalid use of query …

…API)

SQLitePCL.pretty.tests/DatabaseConnectionTests.cs

@@ -633,6 +633,9 @@ public void TestWalCheckpoint()
                 db.Execute("INSERT INTO foo (x) VALUES (1);");
                 db.Execute("INSERT INTO foo (x) VALUES (2);");
 
+                var res = db.Query("Select * from foo limit 1").FirstOrDefault();
+
+
                 int logSize;
                 int framesCheckPointed;
                 db.WalCheckPoint("main", WalCheckPointMode.Full, out logSize, out framesCheckPointed);
@@ -679,6 +682,61 @@ public void TestWalCheckpoint()
             raw.sqlite3__vfs__delete(null, tmpFile, 1);
         }
 
+        [Fact]
+        public void TestHelpfulExceptionOnDisposedResultset()
+        {
+            var tmpFile = GetTempFile();
+            try
+            {
+                using (var db = SQLite3.Open(tmpFile))
+                {
+                    db.ExecuteAll("CREATE TABLE foo (x int);" +
+                                  "INSERT INTO foo (x) VALUES (1);" +
+                                  "INSERT INTO foo (x) VALUES (2);");
+
+
+                    var rows = db.Query("Select * from foo")
+                        // with the call to "ToArray()" the StatementImpl is being disposed
+                        // as any IResultSetValue call basically uses the StatementImpl, it will throw an ObjectDisposedException
+                        .ToArray();
+
+                    Assert.Equal(2, rows.Length);
+
+                    try
+                    {
+                        rows[0][0].ToInt();
+
+                        Assert.True(false, "ObjectDisposedException expected");
+                    }
+                    catch (ObjectDisposedException ex)
+                    {
+                        var contains =
+                            ex.Message.Contains(
+                                "The statement is already disposed. Calling any member of IResultSetValue after the enumeration has completed is not supported. Either access the members using a projection ('.Select(x => x[0].ToInt())') or during enumeration ('foreach(var row in db.Query(...)){row[0].ToInt()}'). Go to https://github.com/bordoley/SQLitePCL.pretty/blob/master/README.md to learn more.");
+                        Assert.True(contains, "helpful message pointing at correct API usage was missing");
+                    }
+                    try
+                    {
+                        rows[0].Count();
+
+                        Assert.True(false, "ObjectDisposedException expected");
+                    }
+                    catch (ObjectDisposedException ex)
+                    {
+                        var contains =
+                            ex.Message.Contains(
+                                "The statement is already disposed. Calling any member of IResultSetValue after the enumeration has completed is not supported. Either access the members using a projection ('.Select(x => x[0].ToInt())') or during enumeration ('foreach(var row in db.Query(...)){row[0].ToInt()}'). Go to https://github.com/bordoley/SQLitePCL.pretty/blob/master/README.md to learn more.");
+                        Assert.True(contains, "helpful message pointing at correct API usage was missing");
+                    }
+                }
+
+            }
+            finally
+            {
+                raw.sqlite3__vfs__delete(null, tmpFile, 1);
+            }
+        }
+
         [Fact]
         public void TestTableColumnMetadata()
         {

SQLitePCL.pretty/Implementation.cs

@@ -195,6 +195,11 @@ public bool IsBusy
             }
         }
 
+        internal bool Disposed
+        {
+            get { return disposed; }
+        }
+
         public void ClearBindings()
         {
             if (disposed) { throw new ObjectDisposedException(this.GetType().FullName); }
@@ -451,6 +456,8 @@ IEnumerator IEnumerable.GetEnumerator() =>
 
     internal sealed class ResultSetImpl : IReadOnlyList<IResultSetValue>
     {
+        private const string Helpmessage =
+            "The statement is already disposed. Calling any member of IResultSetValue after the enumeration has completed is not supported. Either access the members using a projection ('.Select(x => x[0].ToInt())') or during enumeration ('foreach(var row in db.Query(...)){row[0].ToInt()}'). Go to https://github.com/bordoley/SQLitePCL.pretty/blob/master/README.md to learn more.";
         private readonly StatementImpl stmt;
 
         internal ResultSetImpl(StatementImpl stmt)
@@ -459,7 +466,19 @@ internal ResultSetImpl(StatementImpl stmt)
         }
 
         public int Count =>
-            raw.sqlite3_data_count(stmt.sqlite3_stmt);
+            raw.sqlite3_data_count(Stmt.sqlite3_stmt);
+
+        private StatementImpl Stmt
+        {
+            get
+            {
+                if (stmt.Disposed)
+                {
+                    throw new ObjectDisposedException(Helpmessage);
+                }
+                return stmt;
+            }
+        }
 
         public IEnumerator<IResultSetValue> GetEnumerator()
         {
@@ -481,7 +500,7 @@ public IResultSetValue this[int index]
                     throw new ArgumentOutOfRangeException();
                 }
 
-                return stmt.ResultSetValueAt(index);
+                return Stmt.ResultSetValueAt(index);
             }
         }
     }