请教out参数的用法

lxkim 2005-03-04 12:24:09

[StructLayout(LayoutKind.Sequential)]
public class POINT
{
public int x;
public int y;

public POINT()
{
}

public POINT(int x, int y)
{
this.x = x;
this.y = y;
}
}
[DllImport("user32.dll")]
static extern bool GetCursorPos(out POINT lpPoint);

GetCursorPos的参数是out型的

在调用时
POINT lppoint = new POINT(0,0);
GetCursorPos(out lppoint);
label1.Text = lppoint.x.ToString();
label2.Text = lppoint.y.ToString();
为什么会出错？提示未将对象引用设置到对象实例

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egxsun 2005-03-04

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类型要匹配,如果用 class POINT ,就要这样:

[DllImport("user32.dll")]
static extern bool GetCursorPos(POINT lpPoint);
调用时直接:GetCursorPos(lppoint);

struct 类型就要:
[System.Runtime.InteropServices.DllImport( "user32.dll" , EntryPoint="GetCursorPos" )]
internal extern static int GetCursorPos( out POINTAPI lpPoint );
调用时: GetCursorPos(out lppoint);
如果[System.Runtime.InteropServices.DllImport( "user32.dll" , EntryPoint="GetCursorPos" )]
internal extern static int GetCursorPos( ref POINTAPI lpPoint );
调用时: GetCursorPos(ref lppoint);

lxkim 2005-03-04

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上面的改一下：
internal struct POINTAPI
{
internal int x;
internal int y;
}
[System.Runtime.InteropServices.DllImport( "user32.dll" , EntryPoint="GetCursorPos" )]
internal extern static int GetCursorPos( ref POINTAPI lpPoint );
调用时
POINTAPI _POINTAPI = new POINTAPI();
GetCursorPos(ref lppoint);
label1.Text = lppoint.x.ToString();
label2.Text = lppoint.y.ToString();

这样可以,或者用:

internal struct POINTAPI
{
internal int x;
internal int y;
}
[System.Runtime.InteropServices.DllImport( "user32.dll" , EntryPoint="GetCursorPos" )]
internal extern static int GetCursorPos( out POINTAPI lpPoint );
调用时
POINTAPI _POINTAPI = new POINTAPI();
GetCursorPos(out lppoint);
label1.Text = lppoint.x.ToString();
label2.Text = lppoint.y.ToString();

lx1920 2005-03-04

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internal struct POINTAPI
{
internal int x;
internal int y;
}
[System.Runtime.InteropServices.DllImport( "user32.dll" , EntryPoint="GetCursorPos" )]
internal extern static int GetCursorPos( ref POINTAPI lpPoint );
调用时
POINTAPI _POINTAPI = new POINTAPI();
GetCursorPos(out lppoint);
label1.Text = lppoint.x.ToString();
label2.Text = lppoint.y.ToString();