麻烦前辈们，我这是哪错了啊
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Runtime.InteropServices;//声明调用API

namespace WindowsFormsApplication1
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
[DllImport("user32", EntryPoint = "GetForegroundWindow")]

public static extern int GetCapture();

[DllImport("user32", EntryPoint = "GetWindowTextA")]

public static extern string biaoti();

public extern string biaoti(int hwn);
public extern string biaoti(int hwn,string lpString);
public extern string biaoti(int hwn,string lpString,int cc);
private void button1_Click(object sender, EventArgs e)
{

}

private void timer1_Tick(object sender, EventArgs e)
{
textBox1.Text = Convert.ToString(GetCapture());


}
}
}

错误提示

{"未能从程序集“WindowsFormsApplication1, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null”中加载类型“WindowsFormsApplication1.Form1”，因为方法“biaoti”没有实现(没有 RVA)。":"WindowsFormsApplication1.Form1"}

[解决办法]
[DllImport("user32", EntryPoint = "GetForegroundWindow")]

public static extern int GetCapture();

[DllImport("user32", EntryPoint = "GetWindowTextA")]

public static extern string biaoti();

这些都不对。。。

话说你明白调用api的原理么？