更改Windows窗体的背景色

Question

Why can't I change the backcolor of my form this way?

MainForm.BackColor = System.Drawing.Color.Black;

This is what I get in the console:

An object reference is required for the non-static field, method, or property 'System.Windows.Forms.Control.BackColor.get' (CS0120) 

Answer 1

You're using MainForm as if it were a static class. Either make your form static or create an instance of it.

MainForm form = new MainForm();

Then use

form.BackColor = Color.Black;

---

Adding to the comment to your question, stick

 this.BackColor = Color.Black;

inside of a method of your form, and just call that method. Like so.

void changeBackColor(Color color)
{
    this.BackColor = color;
}

That will let you pass a color to the method and changes the BackColor accordingly.

Hope this helps. I'd recommend reading a book about C#. Objects can't be used before being initialized. It's a pretty elementary concept.

Answer 2

Static Classes are classes that cannot be instantiated. Static classes have static methods or static properties (or both). When you use a line like this:

MainForm.BackColor = System.Drawing.Color.Black; // <class name>.<property>

What the C# compiler does first is look for a local class variable called MainForm . Since there was none, it then looked outside your local scope and found your Windows.Form class called MainForm .

It then looked to see if the class MainForm has a static property called BackColor . The compiler then said "Oh look, there is a property called BackColor , but its not static", which is when the compiler complained and threw you the error you experienced.

By changing it to this.BackColor , the compiler knew you wanted to set the background color OF THIS INSTANCE of MainForm, which was this or, by default, form1 :

this.BackColor = System.Drawing.Color.Black; // <this instance>.<property>

And as a side note, the keyword this is not required. Omitting it assumes "this object". You can do this just as well:

BackColor = System.Drawing.Color.Black; // <this instance>.<property>

Hope this makes more sense!