fix: Fix null reference exception in setter that sets DbConnection property of the TDengineCommand class.

src/Data/Client/TDengineCommand.cs

@@ -1,8 +1,8 @@
 using System;
-using System.Collections.Generic;
 using System.Data;
 using System.Data.Common;
-using System.Threading.Tasks;
+using System.Collections.Generic;
+
 using TDengine.Driver;
 
 namespace TDengine.Data.Client
@@ -127,14 +127,7 @@ public override CommandType CommandType
         protected override DbConnection DbConnection
         {
             get => _connection;
-            set
-            {
-                _connection = (TDengineConnection)value;
-                if (_stmt == null && _connection != null)
-                {
-                    _stmt = _connection.client.StmtInit();
-                }
-            }
+            set => _connection = value as TDengineConnection ?? throw new ArgumentException($"The specified connection is not of type {nameof(TDengineConnection)}.");
         }
 
         protected override DbParameterCollection DbParameterCollection => _parameters.Value;
@@ -158,6 +151,11 @@ private IRows Query()
 
         private IRows Statement()
         {
+            if(_stmt == null && _connection != null)
+            {
+                _stmt = _connection.client.StmtInit();
+            }
+
             if (!_isPrepared)
             {
                 _isPrepared = true;

src/Driver/Client/Native/NativeStmt.cs

@@ -80,9 +80,7 @@ public void SetTags(object[] tags)
             }
         }
 
-
-        private TAOS_MULTI_BIND[] GenerateBindList(object[] data, TaosFieldE[] fields, out IntPtr[] needFree,
-            bool isInsert)
+        private TAOS_MULTI_BIND[] GenerateBindList(object[] data, TaosFieldE[] fields, out IntPtr[] needFree, bool isInsert)
         {
             TAOS_MULTI_BIND[] binds = new TAOS_MULTI_BIND[data.Length];
             var needFreePointer = new List<IntPtr>();
@@ -92,7 +90,7 @@ private TAOS_MULTI_BIND[] GenerateBindList(object[] data, TaosFieldE[] fields, o
                 {
                     num = 1
                 };
-                if (data[i] == null)
+                if (data[i] == null || Convert.IsDBNull(data[i]))
                 {
                     bind.buffer_type = (int)TDengineDataType.TSDB_DATA_TYPE_BOOL;
                     IntPtr p = Marshal.AllocHGlobal(TDengineConstant.ByteSize);
@@ -471,7 +469,6 @@ private TAOS_MULTI_BIND GenerateBindColumn(Array array, TaosFieldE field)
             }
         }
 
-
         public void AddBatch()
         {
             var code = NativeMethods.StmtAddBatch(_stmt);

test/Data.Tests/TDengineCommandTests.cs

@@ -65,6 +65,47 @@ public void Dispose()
         }
 
         [Fact]
+        public void SetConnection_DoesNotThrowException()
+        {
+            //CASE 1: Set an already open connection.
+            using(var command = new TDengineCommand())
+            {
+                command.CommandText = "SELECT * FROM t";
+                var ex = Record.Exception(() => command.Connection = _connection);
+                Assert.Null(ex);
+            }
+
+            //CASE 2: Set an connection that is not yet open.
+            using(var command = new TDengineCommand())
+            {
+                using(var connection = new TDengineConnection("username=root;password=taosdata"))
+                {
+                    command.CommandText = "SELECT * FROM t";
+                    var ex = Record.Exception(() => command.Connection = connection);
+                    Assert.Null(ex);
+                }
+            }
+        }
+
+        [Fact]
+        public void ExecuteNonQuery_WithDBNull()
+        {
+            using(var command = new TDengineCommand(_connection))
+            {
+                command.CommandText = "create table if not exists t_dbnull (ts timestamp,v int,description nchar(100))";
+                command.ExecuteNonQuery();
+                command.CommandText = "INSERT INTO t_dbnull VALUES (?,?,?)";
+                command.Parameters.AddWithValue(DateTime.Now);
+                command.Parameters.AddWithValue(123);
+                command.Parameters.AddWithValue(DBNull.Value);
+
+                int affectedRows = command.ExecuteNonQuery();
+
+                Assert.Equal(1, affectedRows);
+            }
+        }
+
+		[Fact]
         public void ExecuteNonQuery_WithValidCommand_ReturnsAffectedRows()
         {
             using (var command = new TDengineCommand(_connection))