如何将字符串转换为nvarchar？

Question

我有一个文本框，我想将输出结果保存到 Unicode （SQL Server中的nvarchar），对于我使用Entity Framework的保存操作，我的代码工作正常，我的问题只是将文本框结果转换为Unicode，因为当我保存结果时它保存为“？”在Sql Server中。 注意：我使用阿拉伯字母。，这是我的代码和列定义： 代码：

//Button add new car into database
    private void BtnAddNewCar_Click(object sender, EventArgs e)
    {
        v.brand = TxBoxBrand.Text;
        v.car_number = TxBoxCarNumber.Text;//Text Box that i want to convert to Unicode (Arabic letters)
        v.color = CmBoxColorPicker.Text;
        v.model = TxBoxModel.Text;
        v.picture = ImageToByteArray(filename);
        v.cin_driver= CmBoxCarOwner.Text;
        context.Vehicles.Add(v);
        context.SaveChanges();
        NotifyWelcome.ShowBalloonTip(1000, "The vehicle " + TxBoxBrand.Text + " was successfully added", " Have a ice drive", ToolTipIcon.Info);
        CarClear();
        VLoad.Enabled = true;
        CmVLoad.Enabled = false;
        MyMessageBox.ShowBox("The vehicle " + TxBoxBrand.Text + " as successfully added", "Notify");
    }

列定义：

// TxBoxCarNumber
        // 
        this.TxBoxCarNumber.Font = new System.Drawing.Font("Ubuntu", 9F, System.Drawing.FontStyle.Bold, System.Drawing.GraphicsUnit.Point, ((byte)(0)));
        this.TxBoxCarNumber.Location = new System.Drawing.Point(272, 30);
        this.TxBoxCarNumber.Name = "TxBoxCarNumber";
        this.TxBoxCarNumber.Size = new System.Drawing.Size(142, 21);
        this.TxBoxCarNumber.TabIndex = 0;
        this.TxBoxCarNumber.TabStop = false;