Reputation: 12538

ToString on null string

Why does the second one of these produce an exception while the first one doesn't?

string s = null;
MessageBox.Show(s);
MessageBox.Show(s.ToString());

Updated - the exception I can understand, the puzzling bit (to me) is why the first part doesn't show an exception. This isn't anything to do with the Messagebox, as illustrated below.

Eg :

string s = null, msg;
msg = "Message is " + s; //no error
msg = "Message is " + s.ToString(); //error

The first part appears to be implicitly converting a null to a blank string.

Upvotes: 39

Answers (10)

Ogglas

Reputation: 69968

As you say yourself it has nothing to do with MessageBox, it depends on the implementation of the ToString() method.

For example looking at struct Nullable<T> where T : struct from System.Runtime that a nullable enum uses as example:

[Serializable]
[NonVersionable] // This only applies to field layout
[System.Runtime.CompilerServices.TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
public partial struct Nullable<T> where T : struct

It's implementation of ToString() looks like this for .NET 7:

public override string? ToString() => hasValue ? value.ToString() : "";

Therefore the following code gives an exception on MessageBox.Show(s.ToString()); even though ToString() is called on a null object for tc.TestEnum.

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        var tc = new TestClass();
        MessageBox.Show(tc.TestEnum.ToString());

        string s = null;
        MessageBox.Show(s);
        MessageBox.Show(s.ToString());
    }
}

public enum TestEnum
{
    None = 0
}

public class TestClass
{

    public TestEnum? TestEnum { get; set; }

}

System.NullReferenceException: 'Object reference not set to an instance of an object.'

Looking at class String from System.Runtime:

[Serializable]
[NonVersionable] // This only applies to field layout
[System.Runtime.CompilerServices.TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
public sealed partial class String : IComparable, IEnumerable, IConvertible, IEnumerable<char>, IComparable<string?>, IEquatable<string?>, ICloneable

Here we have another implementation that will throw the error you see:

// Returns this string.
public override string ToString()
{
    return this;
}

However starting with .NET 6< <Nullable>enable</Nullable> is enabled by default for new projects.

https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references

The code above will therefore give you two warnings:

string s = null; -> CS8600 Converting null literal or possible null value to non-nullable type.

MessageBox.Show(s.ToString()); -> CS8602 Dereference of a possibly null reference

Rewrite the code like this and the errors disappear:

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        var tc = new TestClass();
        MessageBox.Show(tc.TestEnum.ToString());

        string? s = null;
        MessageBox.Show(s);
        if (s is not null)
        {
            MessageBox.Show(s.ToString());
        }
        
    }
}

public enum TestEnum
{
    None = 0
}

public class TestClass
{

    public TestEnum? TestEnum { get; set; }

}

Or simply using the null-conditional operator ?. like @mr R writes:

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        var tc = new TestClass();
        MessageBox.Show(tc.TestEnum.ToString());

        string? s = null;
        MessageBox.Show(s);
        MessageBox.Show(s?.ToString());
    }
}

public enum TestEnum
{
    None = 0
}

public class TestClass
{

    public TestEnum? TestEnum { get; set; }

}

Upvotes: 0

mr R

Reputation: 1126

ToString() can't operate on null reference of s vairable

The shortest way

obj?.ToString()

Another correct ways

obj?.ToString() ?? string.Empty  
obj?.ToString() ?? "default string value"

https://www.informit.com/articles/article.aspx?p=2421572

Upvotes: 2

RooiWillie

Reputation: 2228

As this question ranks quite high on Google for a search for "c# toString null", I would like to add that the Convert.ToString(null) method would return ~~an empty~~ a null string, which is ignored by the messagebox.

However, just to reaffirm the other answers, you can use string.Concat("string", null) in this example.

Edit - modified answer in line with HeyJude's comment below. As pointed out, a method like Convert.ToString(null).Length will throw an exception.

Upvotes: 15

James

Reputation: 71

Behind the scenes concat is being called in your follow up question / update E.g

string snull = null;

string msg = "hello" + snull;

// is equivalent to the line below and concat handles the null string for you.
string msg = String.Concat("hello", snull);

// second example fails because of the toString on the null object
string msg = String.Concat("hello", snull.ToString());

//String.Format, String.Convert, String.Concat all handle null objects nicely.

Upvotes: 7

Hans Passant

Reputation: 941465

It is because MessageBox.Show() is implemented with pinvoke, it calls the native Windows MessageBox() function. Which doesn't mind getting a NULL for the lpText argument. The C# language has much stricter rules for pure .NET instance methods (like ToString), it always emits code to verify that the object isn't null. There's some background info on that in this blog post.

Upvotes: 14

user586138

Reputation: 1

Probably the Show method handles a null value and just shows nothing. The second use of s - s.ToString() fails because you there is no ToString method to run.

Upvotes: 0

gbvb

Reputation: 886

Because, the second call is expecting an object of "s" to satisfy a ToString() method request. so, before .Show() is called, the s.ToString() would failed with an attempt to call a method.

Interestingly, While .Show() is implemented correctly, many such methods expect non null instances to be passed in. Usually, that is when you use a NullObject pattern so that the caller should not have to deal with this kind of behavior.

Upvotes: 0