CODEWITHSUNDEEP
c#
Michael Jolley
Michael Jolley

Reputation: 368

Why isn't my New() firing?

I'm having problems with the code below. Whenever I create a new instance of the class below and call "Select" I get an object reference not set to an instance of an object on the "this.db"

namespace SPI {

    class CompaniesDB
    {
        private DataContainer db;

        public void New() {
            this.db = new DataContainer();
        }
        public Company Select(int companyID) {
            return this.db.Company_Get(companyID).SingleOrDefault();
        }
    }
}

Can someone point me at why my "New()" doesn't seem to be creating a new object?

I'm relatively new to C#.

Upvotes: 0

Views: 209

Answers (4)

fletcher
fletcher

Reputation: 13780

In this case, the DataContainer constructor requires no arguments and instantion isn't affected by a variable within your New function, therefore you could do away with that function entirely. You could instantiate the DataContainer upon declaration of the private db variable:

namespace SPI {

    class CompaniesDB
    {
        private DataContainer db = new DataContainer();

        public Company Select(int companyID) {
            return this.db.Company_Get(companyID).SingleOrDefault();
        }
    }
}

Edit, further information as per Mike Jolley's request:

This is really down to preference, both solutions will work and are safe.

You could use the following code:

namespace SPI { 

    class CompaniesDB 
    { 
        private DataContainer db;

        public CompaniesDB()
        {
            db = new DataContainer();
        }

        public Company Select(int companyID) { 
            return this.db.Company_Get(companyID).SingleOrDefault(); 
        } 
    } 
}

This code is good, you are creating an instance of DataContainer whose reference will be assigned to the db variable. You are ensuring that the db variable contains a reference when you want to start using it.

However, this instantiation is always the same, therefore you could get rid of the assignment within the constructor and just create a normal DataContainer by default. This would also ensure that the variable always contains a reference to a DataContainer upon instantiation of a CompaniesDB object:

namespace SPI { 

    class CompaniesDB 
    { 
        private DataContainer db = new DataContainer(); 

        public Company Select(int companyID) { 
            return this.db.Company_Get(companyID).SingleOrDefault(); 
        } 
    } 
}

The need for a constructor would be clear if the DataContainer constructor took an argument:

namespace SPI { 

    class CompaniesDB 
    { 
        private DataContainer db;

        public CompaniesDB(string name)
        {
            db = new DataContainer(name);
        }

        public Company Select(int companyID) { 
            return this.db.Company_Get(companyID).SingleOrDefault(); 
        } 
    } 
}

The introduction of an argument would mean that it would be better to create the instance of DataContainer in the constructor.

Upvotes: 0

Aryabhatta
Aryabhatta

Reputation:

You don't name a constructor as New. You name it with the class Name.

Try

namespace SPI { 

    class CompaniesDB 
    { 
        private DataContainer db; 

        public CompaniesDB() { 
            this.db = new DataContainer(); 
        } 
        public Company Select(int companyID) { 
            return this.db.Company_Get(companyID).SingleOrDefault(); 
        } 
    } 
}

MSDN page on constructors: http://msdn.microsoft.com/en-us/library/ms173115.aspx

Upvotes: 11

ale
ale

Reputation: 519

Do you intend for New() to be your constructor? If so, the syntax for constructors in C# asks for the name of the class. So your constructor should be something like:

public CompaniesDB()
{
    this.db = new DataContainer();
}

hope that helps.

Upvotes: 1

Chris Dennett
Chris Dennett

Reputation: 22731

Rename the New method to CompaniesDB? Also remove the 'void' modifier.

Upvotes: 1

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