回答

[DllImport("User32.dll",EntryPoint="FindWindow")]
private static extern IntPtr FindWindow(string lpClassName,string lpWindowName);
IntPtr hWnd = FindWindow(null,"微信");

捕获窗体内的文本，先看看相关的api

// 查找窗口
[DllImport("user32.dll", EntryPoint = "FindWindow", SetLastError = true)]
public static extern IntPtr FindWindow(string lpClassName, string lpWindowName);

// 遍历窗口的所有子窗口，通过CallBack回调
[DllImport("user32.dll")]
public static extern int EnumChildWindows(IntPtr hWndParent, CallBack lpfn, int lParam);
public delegate bool CallBack(IntPtr hwnd, int lParam);

// 获取窗口的类名
[DllImport("user32.dll", SetLastError = true, CharSet = CharSet.Auto)]
static extern int GetClassName(IntPtr hWnd, StringBuilder lpClassName, int nMaxCount);

// 判断窗口是否可见
[DllImport("user32.dll")]
public static extern bool IsWindowVisible(IntPtr hWnd);

// 获取窗口文本长度
[DllImport("user32.dll")]
public static extern int GetWindowTextLength(IntPtr hWnd);

// 获取窗口文本，文本会塞入StringBuilder中，需要指明字符串最大长度nMaxCount
[DllImport("User32.dll", EntryPoint = "GetWindowText")]
private static extern int GetWindowText(IntPtr hwnd, StringBuilder lpString, int nMaxCount);

// 给窗口发送消息
[DllImport("user32.dll", EntryPoint = "SendMessageA")]
public static extern int SendMessage(IntPtr hwnd, int wMsg, int wParam, int lParam);

// 给窗口发送消息，事件返回的数据通过Byte[]数组获得
[DllImport("user32.dll", EntryPoint = "SendMessageA")]
public static extern int SendMessage(IntPtr hwnd, int wMsg, int wParam, Byte[] lParam);

using System;
using System.Runtime.InteropServices;
using System.Text;


public partial class WindowApiHelper 
{
    /// <summary>
    /// 尝试查找Error窗口并取出窗口文本
    /// </summary>
    /// <returns></returns>
    public static string TryFindErrorWindowText()
    {
        string errorText = "";
        // 查找标题为Error的窗口
        IntPtr mainHandle = FindWindow(null, "Error");
        if (mainHandle != IntPtr.Zero)
        {
            // 枚举子窗体，查找控件句柄
            int i = EnumChildWindows(mainHandle, (h, l) =>
            {
                StringBuilder sbr = new StringBuilder();
                GetClassName(h, sbr, 255);
                string classname = sbr.ToString();
                // 获取Edit子窗口
                if ("Edit" == classname)
                {
                    // 是否可见
                    if (IsWindowVisible(h))
                    {
                        // 取出窗口文本
                        int textLen;
                        textLen = SendMessage(h, WM_GETTEXTLENGTH, 0, 0);
                        Byte[] byt = new Byte[textLen];
                        SendMessage(h, WM_GETTEXT, textLen + 1, byt);
                        errorText = Encoding.Default.GetString(byt);
                        // 关闭Error窗口
                        // SendMessage(h, WM_CLOSE , 0, 0);
                    }
                }

                return true;
            }, 0);
        }
        return errorText;
    }

    /*--Windows API------------------------------------------------------------------------------------*/

    [DllImport("user32.dll", EntryPoint = "FindWindow", SetLastError = true)]
    public static extern IntPtr FindWindow(string lpClassName, string lpWindowName);

    [DllImport("user32.dll")]
    public static extern int EnumChildWindows(IntPtr hWndParent, CallBack lpfn, int lParam);
    public delegate bool CallBack(IntPtr hwnd, int lParam);

    [DllImport("user32.dll", SetLastError = true, CharSet = CharSet.Auto)]
    static extern int GetClassName(IntPtr hWnd, StringBuilder lpClassName, int nMaxCount);

    [DllImport("user32.dll")]
    public static extern bool IsWindowVisible(IntPtr hWnd);

    [DllImport("user32.dll")]
    public static extern int GetWindowTextLength(IntPtr hWnd);

    [DllImport("User32.dll", EntryPoint = "GetWindowText")]
    private static extern int GetWindowText(IntPtr hwnd, StringBuilder lpString, int nMaxCount);

    [DllImport("user32.dll", EntryPoint = "SendMessageA")]
    public static extern int SendMessage(IntPtr hwnd, int wMsg, int wParam, int lParam);

    [DllImport("user32.dll", EntryPoint = "SendMessageA")]
    public static extern int SendMessage(IntPtr hwnd, int wMsg, int wParam, Byte[] lParam);

    const int WM_GETTEXT = 0x000D;
    const int WM_GETTEXTLENGTH = 0x000E;
    const int WM_CLOSE = 0x10;

    /*--Windows API------------------------------------------------------------------------------------*/
}