Assign value to variable

I have this variable, it's an instance of a Interface of a class in business layer. I need to send messages since data access layer, to business layer and finally to presentation layer. I my class "LogBinaryWriter" in data access I have this:

public class LogBinaryWriter : ILogBinaryWriter
{
   private readonly IImageLogBuilder _imageLogBuilder;

   public void WriteFrameCodes(string filePath, int logSelected)
   {
     var fileE开发者_运维技巧xists = FileExists(binaryFilePath);
     if (fileExists == true)
     {
        _imageLogBuilder.displayMessage("The file " + binaryFileName + " exist.             Dou you want overwrite it? (Y/N)");
     }
   }
}

I have a message: "the value _imageLogBuilder is never assigned and will always be null"

How can I fix this?

Instantiate an instance of IImageLogBuilder in your constructor for LogBinaryWriter and assign it to _imageLogBuilder. You would have to do it in the constructor since you have _imageLogBuilder marked as readonly.

For example, assuming you have a class called MyImageLogBuilder that implements IImageLogBuilder:

public LogBinaryWriter()
{
    _imageLogBuilder = new MyImageLogBuilder();
}

You could also overload the constructor so you can pass in the IImageLogBuilder you want to use (lookup constructor injection for more info on this pattern):

public LogBinaryWriter(IImageLogBuilder imageLogBuilder)
{
    _imageLogBuilder = imageLogBuilder;
}

Remember, you will need a class that implements the IImageLogBuilder interface to be able to create a new instance and assign it to the _imageLogBuilder variable. For example:

public interface IImageLogBuilder
{
    void Log(string message);
}

//The class below IMPLEMENTS the IImageLogBuilder interface
public class MyImageLogBuilder : IImageLogBuilder
{
   //Implement IImageLogBuilder methods here
   public void Log(string message)
   {
      //Log message
   }
}

If you had something like the classes defined above then you could the following in the LogBinaryWriter constructor and you would no longer get the null reference error.

public LogBinaryWriter()
{
    _imageLogBuilder = new MyImageLogBuilder();
}

You need to assign an instance of a class that implements IImageLogBuilder interface to _imageLogBuilder field.

Right now your field will always have a value of null.

For example:

private readonly IImageLogBuilder _imageLogBuilder = new ImageLogBuilder();

perhaps initialize _imageLogBuilder in constructor ?

public class LogBinaryWriter : ILogBinaryWriter
{
   private readonly IImageLogBuilder _imageLogBuilder;

   public LogBinaryWriter(IImageLogBuilder imageLogBuilder)
   {
      _imageLogBuilder = imageLogBuilder;
   }
....


}

You never set the value of the _imageLogBuilder variable. And since you marked it as readonly, the only place it can be set is in a field initializer, or in a constructor.

Did you mean to do something like this, perhaps?

private readonly IImageLogBuilder _imageLogBuilder = // get the value from somewhere else, or make a new one