c# - 一定秒数后出现空引用异常

Question

我在注释中指出的行中遇到空引用异常。有谁知道为什么会这样？

var form = Form.ActiveForm as Form1;
var doSave = MessageBox.Show("Would you like to save this measurement?",
                             "Save Measurement",
                             MessageBoxButtons.YesNo,
                             MessageBoxIcon.Question);

if (doSave == DialogResult.Yes) // User wants to save the current measurement.
{
    curName = ShowDialog("Measurement Name", "Save Measurement");
    // ERROR HERE **** 
    int ret = form.databaseClass.GetFirstSoundReadyTestOccurrence(form.testNumber.Text);

Answer 1

我敢打赌这是 Form.ActiveForm 不是 Form1 类型的情况。

这应该很容易重现。在进入此代码之前，只需确保您的事件表单不是 Form1 类型。换句话说...确保运行此代码时其他一些表单处于事件状态。

下面的编辑将在发生这种情况时修复，但您确实需要一种更可靠的方式来获取表单。在不了解您的设计的情况下，很难推荐更好的获取表单的方法。

var form = Form.ActiveForm as Form1;
if (form != null)
{
var doSave = MessageBox.Show("Would you like to save this measurement?",
                             "Save Measurement",
                             MessageBoxButtons.YesNo,
                             MessageBoxIcon.Question);

if (doSave == DialogResult.Yes) // User wants to save the current measurement.
{
    curName = ShowDialog("Measurement Name", "Save Measurement");
    // ERROR HERE **** 
    int ret = form.databaseClass.GetFirstSoundReadyTestOccurrence(form.testNumber.Text);
}
}