Error and solution for C# Static method cannot access non-static members

Error: Object reference required

The following code snippet demonstrates this problem:

namespace WindowsApplication1
{
    public partial class Form1 : Form
    {
        ...

        private static void SumData(object state)
        {
            ...
            setTextboxText(result); // 错误：非静态字段、方法或属性
        }
    }
}

Cause of the problem

Error message indicates that the static method SumData attempts to call a non-static member setTextboxText. A static method can only access static members, while accessing non-static members requires a reference to the object of the class to which it belongs.

Solutions

There are several ways to fix this error:

1. Set the setTextboxText method to static:

public static void setTextboxText(int result)

However, if the setTextboxText method requires access to the instance variable, it cannot be set to static.

2. static singleton call setTextboxText:]

class Form1
{
    public static Form1 Instance;   // 单例

    ...

    private static void SumData(object state)
    {
        ...
        Instance.setTextboxText(result);
    }
}

Form1, Instance needs to be set to the current instance: Instance = this;.

3. Create a Form1 In the calling method:

private static void SumData(object state)
{
    ...
    Form1 frm1 = new Form1();
    frm1.setTextboxText(result);
}

Form1 already exists, this method may not work.

4. SumDataSet the call method SumData as a non-static instance method (belongs to Form1):

private void SumData(object state)
{
    ...
    setTextboxText(result);
}

This is usually the best solution because it keeps the code encapsulation and maintainability.

For more information, please refer to the MSDN documentation.