C# Create new object with property values in constructor return Null

Question

When a new object of class Foo is created, the constructor is supposed to create a new folder and a new file based on the object properties. But I get NullException (param: path2)?

I found that the object properties has Null value when the constructor is called. But I gave the properties values when I created the object? What am I missing?

My Foo class:

public class Foo
{
    public string Bar { get; set; }
    public string Baz { get; set; }
    public string Source { get { return Path.Combine(Qux, Baz, Bar); } }
    private string Qux { get { return Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData); } }

    public Foo()
    {
        // Use property values to find or create Directory and File
        if (!Directory.Exists(Path.Combine(Qux, Baz))) Directory.CreateDirectory(Path.Combine(Qux, Baz));
        if (!File.Exists(Source)) File.Create(Source);
    }
}

In my Main class:

// Create a new Foo object with following property values
Foo foo = new Foo { Baz = "corge", Bar = "grault" };

Possible duplicate of What is a NullReferenceException, and how do I fix it? — Dmitry
– Dmitry, Commented Nov 14, 2016 at 15:34
With that syntax, the constructor runs before the properties are assigned to. Since you're using them in the constructor's body, you're out of luck. Maybe make them constructor arguments instead? — Frédéric Hamidi
– Frédéric Hamidi, Commented Nov 14, 2016 at 15:35
Possible duplicate of stackoverflow.com/questions/17327266/… - Constructor vs Object Initializer Precedence in C# — KMoussa
– KMoussa, Commented Nov 14, 2016 at 15:36

David · Accepted Answer · 2016-11-14 15:36:40Z

9

But I gave the properties values when I created the object?

No you didn't. (Though admittedly it may be a little unintuitive if you're new to the syntax.)

The code is expecting those to be supplied in the constructor. But you have a parameterless constructor:

public Foo()
{
    //...
}

So when that constructor executes those properties haven't been set and have their default values.

Add the parameters to the constructor itself:

public Foo(string baz, string bar)
{
    Baz = baz;
    Bar = bar;
    //...
}

And then supply them to the constructor:

new Foo("corge", "grault")

What you're doing here:

Foo foo = new Foo { Baz = "corge", Bar = "grault" };

Is the equivalent of this:

Foo foo = new Foo();
foo.Baz = "corge";
foo.Bar = "grault";

The constructor is being called first, before the parameters are set.

answered Nov 14, 2016 at 15:36

David

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