看了这篇文章:://blog.csdn.net/aroc_lo/article/details/38387379
才知道,CTRA+A == 1,我用的C++Builder,下面是改进的代码:

void __fastcall TfrmMain::Memo1KeyPress(TObject *Sender, wchar_t &Key)
{
    if(Key == 1){
        Memo1->SelectAll();
        Key = 0;  // 设置 Key 为 0,不然会滴滴响
    }
}