How to use output parameter with StringBuilder?

Question

I pass a stringbuild out variable to function which parameter type is out stringbuild, but compiler tel me

"Before leave function, You must assign value to sbValue"

    public void FuncOutParamString(out StringBuilder sbValue)
    {
        sbValue.Append("wold!");
    }

    private void btnOutParam_Click(object sender, RoutedEventArgs e)
    {
        Console.WriteLine("=================== Out Parameter ===================");
        // Declare a variable but don't assign a value to it ******************************
        int y = 0;

        // Pass it in as an output parameter, even though its value is unassigned
        FuncOutParamInt(out y);

        // It's now assigned a value, so we can write it out:
        Console.WriteLine(y);

        // Declare a variable but don't assign a value to it ******************************
        StringBuilder sb = new StringBuilder();
        sb.Append("Hello ");
        FuncOutParamString(out sb);

        Console.WriteLine(sb);
    }

Sorry, I need 10 reputation to post image

Answer 1

[out] means to be created. [class] is a reference type.

public void FuncOutParamString(StringBuilder sbValue)
{
    sbValue.Append("wold!");
}

public void FuncOutParamStringRef(ref StringBuilder sbValue)
{
    //if(...)
        sbValue = new StringBuilder();
}

public void FuncOutParamStringOut(out StringBuilder sbValue)
{
    sbValue = new StringBuilder();
}