函数Conver.ToDouble，出现错误

如何解决函数Conver.ToDouble，出现错误

使用Convert返回错误：

严重性代码说明文件项目字符串抑制状态 不允许使用类型名称的错误（活动）E0254 C：\ Users \ senya \ source \ repos \ Project6 \ Project6 \ MyForm.h Project6 174 错误C2275系统：：转换：将该类型无效地用作表达式C：\ Users \ senya \ source \ repos \ Project6 \ Project6 \ MyForm.h Project6 174

代码：

#pragma once

namespace Project6 {

    using namespace System;
    using namespace System::ComponentModel;
    using namespace System::Collections;
    using namespace System::Windows::Forms;
    using namespace System::Data;
    using namespace System::Drawing;

    /// <summary>
    /// Сводка для MyForm
    /// </summary>
    public ref class MyForm : public System::Windows::Forms::Form
    {
    public:
        MyForm(void)
        {
            InitializeComponent();
            //
            //TODO: добавьте код конструктора
            //
        }

    protected:
        /// <summary>
        /// Освободить все используемые ресурсы.
        /// </summary>
        ~MyForm()
        {
            if (components)
            {
                delete components;
            }
        }
    private: System::Windows::Forms::Label^ label1;
    protected:
    private: System::Windows::Forms::TextBox^ textBox1;
    private: System::Windows::Forms::Label^ label2;
    private: System::Windows::Forms::TextBox^ textBox2;
    private: System::Windows::Forms::Label^ label3;
    private: System::Windows::Forms::ComboBox^ comboBox1;
    private: System::Windows::Forms::Button^ button1;
    private: System::Windows::Forms::Button^ button2;
    private: System::Windows::Forms::TextBox^ textBox3;

    private:
        /// <summary>
        /// Обязательная переменная конструктора.
        /// </summary>
        System::ComponentModel::Container ^components;

#pragma region Windows Form Designer generated code
        /// <summary>
        /// Требуемый метод для поддержки конструктора — не изменяйте 
        /// содержимое этого метода с помощью редактора кода.
        /// </summary>
        void InitializeComponent(void)
        {
            this->label1 = (gcnew System::Windows::Forms::Label());
            this->textBox1 = (gcnew System::Windows::Forms::TextBox());
            this->label2 = (gcnew System::Windows::Forms::Label());
            this->textBox2 = (gcnew System::Windows::Forms::TextBox());
            this->label3 = (gcnew System::Windows::Forms::Label());
            this->comboBox1 = (gcnew System::Windows::Forms::ComboBox());
            this->button1 = (gcnew System::Windows::Forms::Button());
            this->button2 = (gcnew System::Windows::Forms::Button());
            this->textBox3 = (gcnew System::Windows::Forms::TextBox());
            this->SuspendLayout();
            // 
            // label1
            // 
            this->label1->AutoSize = true;
            this->label1->Font = (gcnew System::Drawing::Font(L"Microsoft Sans Serif",12));
            this->label1->Location = System::Drawing::Point(12,43);
            this->label1->Name = L"label1";
            this->label1->Size = System::Drawing::Size(187,20);
            this->label1->TabIndex = 0;
            this->label1->Text = L"Введите первое число:";
            // 
            // textBox1
            // 
            this->textBox1->Location = System::Drawing::Point(16,77);
            this->textBox1->Name = L"textBox1";
            this->textBox1->Size = System::Drawing::Size(173,20);
            this->textBox1->TabIndex = 1;
            // 
            // label2
            // 
            this->label2->AutoSize = true;
            this->label2->Font = (gcnew System::Drawing::Font(L"Microsoft Sans Serif",12));
            this->label2->Location = System::Drawing::Point(219,43);
            this->label2->Name = L"label2";
            this->label2->Size = System::Drawing::Size(187,20);
            this->label2->TabIndex = 2;
            this->label2->Text = L"Введите первое число:";
            // 
            // textBox2
            // 
            this->textBox2->Location = System::Drawing::Point(223,77);
            this->textBox2->Name = L"textBox2";
            this->textBox2->Size = System::Drawing::Size(173,20);
            this->textBox2->TabIndex = 3;
            // 
            // label3
            // 
            this->label3->AutoSize = true;
            this->label3->Font = (gcnew System::Drawing::Font(L"Microsoft Sans Serif",12));
            this->label3->Location = System::Drawing::Point(12,119);
            this->label3->Name = L"label3";
            this->label3->Size = System::Drawing::Size(168,20);
            this->label3->TabIndex = 4;
            this->label3->Text = L"Выберите операцию:";
            // 
            // comboBox1
            // 
            this->comboBox1->FormattingEnabled = true;
            this->comboBox1->Items->AddRange(gcnew cli::array< System::Object^  >(4) { L"+",L"-",L"*",L"/" });
            this->comboBox1->Location = System::Drawing::Point(16,165);
            this->comboBox1->Name = L"comboBox1";
            this->comboBox1->Size = System::Drawing::Size(150,21);
            this->comboBox1->TabIndex = 5;
            // 
            // button1
            // 
            this->button1->Location = System::Drawing::Point(16,260);
            this->button1->Name = L"button1";
            this->button1->Size = System::Drawing::Size(254,69);
            this->button1->TabIndex = 6;
            this->button1->Text = L"Расчитать";
            this->button1->UseVisualStyleBackColor = true;
            this->button1->Click += gcnew System::EventHandler(this,&MyForm::button1_Click);
            // 
            // button2
            // 
            this->button2->Location = System::Drawing::Point(336,260);
            this->button2->Name = L"button2";
            this->button2->Size = System::Drawing::Size(254,69);
            this->button2->TabIndex = 7;
            this->button2->Text = L"Сбросить";
            this->button2->UseVisualStyleBackColor = true;
            // 
            // textBox3
            // 
            this->textBox3->Location = System::Drawing::Point(31,220);
            this->textBox3->Name = L"textBox3";
            this->textBox3->Size = System::Drawing::Size(436,20);
            this->textBox3->TabIndex = 8;
            // 
            // MyForm
            // 
            this->AutoScaleDimensions = System::Drawing::SizeF(6,13);
            this->AutoScaleMode = System::Windows::Forms::AutoScaleMode::Font;
            this->ClientSize = System::Drawing::Size(617,445);
            this->Controls->Add(this->textBox3);
            this->Controls->Add(this->button2);
            this->Controls->Add(this->button1);
            this->Controls->Add(this->comboBox1);
            this->Controls->Add(this->label3);
            this->Controls->Add(this->textBox2);
            this->Controls->Add(this->label2);
            this->Controls->Add(this->textBox1);
            this->Controls->Add(this->label1);
            this->Name = L"MyForm";
            this->Text = L"MyForm";
            this->ResumeLayout(false);
            this->PerformLayout();

        }
#pragma endregion
    private: System::Void button1_Click(System::Object^ sender,System::EventArgs^ e) {
        double a,b;
        a = Convert.ToDouble(textBox1->Text);
    }
};
}