Why Does 'CS0120: An object reference is required...' Occur When Calling a Non-Static Method from a Static Method?

Error: "CS0120: An object reference is required for the nonstatic field, method, or property 'foo'"

Scenario:

Consider the following code:

private static void SumData(object state)
{
    // Calling a non-static member from a static method
    setTextboxText(result);
}

Explanation:

The error "CS0120" occurs when you attempt to access a non-static member (field, method, or property) from a static context. In this case, the setTextboxText method is a non-static member of the Form1 class, and it cannot be accessed from the static SumData method.

Possible Solutions:

• Make the Non-Static Member Static:

static void setTextboxText(int result)
{
    // Implementation details
}

• Use a Static Singleton of Form1:

class Form1
{
    public static Form1 It;

    public Form1()
    {
        It = this;
    }

    private static void SumData(object state)
    {
        Form1.It.setTextboxText(result);
    }
}

• Pass an Instance of Form1 to the Calling Method:

private static void SumData(Form1 form, object state)
{
    form.setTextboxText(result);
}

• Make the Calling Method Non-Static:

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

Additional Information:

• Static members can only access other static members.
• Non-static members can access both static and non-static members.
• If an instance of the class is not available during the call to a non-static member, you can create a new instance or use static singletons to access the non-static member.
• Refer to MSDN for more information about this error.

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