System.Nullreferenceexception in an attribute of type Stringbuilder

Question

Why error occurs? How to fix?

Model:

public class modelExemplo
{
        public StringBuilder listNewsletter { get; set; }

}

Controller:

public ActionResult funcaodeteste()
{
    modelExemplo obj = new modelExemplo();
    obj.listNewsletter.AppendLine("teste1");
    obj.listNewsletter.AppendLine("teste2");    
}

Answer 1

This happens because StringBuilder is a class non-static, that is to say, needs to be instantiated (initialized). Do this in your constructor.

public class modelExemplo
{
        public modelExemplo(){
            listNewsletter = new StringBuilder();
        }

        public StringBuilder listNewsletter { get; set; }

}

Answer 2

Property object needs to be initialized:

public StringBuilder listNewsletter { get; set; } = new StringBuilder();

If you use an older version of C# you need to do this in a constructor, basic example:

public modelExemplo() {
    listNewsletter = new StringBuilder();
}

I put in the Github for future reference.

Declaring a variable of a type is different from instantiating an object for it. Without an existing object any operation on it will fail.

Answer 3

listNewsletter is null, so listNewsletter.AppendLine will fire a NullReferenceException

You can fix this by initiating listNewsletter in the builder of his class:

public class modelExemplo
{
    public StringBuilder listNewsletter { get; set; }

    public modelExemplo(){
        listNewsletter = new StringBuilder();
    }

}

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