Use the “?” operator on methods when possible. If a variable can be null, then check for null and handle it appropriatelyģ. Initialize variables with valid values.Ģ. Tips to Prevent Null Reference Exceptionsġ. Good exception handling best practices are critical. Pretend every database call is going to fail, every field is going to have messed up data in it. Developers should always assume that everything is invalid and be very defensive in their code.
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The good news is that a lot of null reference errors can be avoided by adding additional logic and code to ensure objects are not null before trying to use them. I think every new programmer needs a tattoo that says it. I call it the golden rule of programming. The Golden Rule of Programmingįor years I have had a saying that I say to my team all the time. KA-BOOM: Object reference not set to an instance of an object. Randomly records get queried, and the code didn’t account for that new field is null. For example, you add a new field to your database and don’t populate default values for every record. Some of the most common causes are settings, database calls, or API-type calls not returning expected values.
Simple Examples of Null Values Causing Problems Adding “? new List()” prevents the “Object reference not set to an instance of an object” exception.įoreach (var value in values ? new List()) The following code throws an exception without the null coalescing. It works great for providing a default value for a variable that is null. Int? count = customerList?.Orders?.Count() // null if customerList, the first customer, or Orders is null Use Null Coalescing to Avoid NullReferenceExceptionsĪnother great feature is null coalescing, which is the “?” operator. Int? length = customerList?.Length // null if customerList is nullĬustomer first = customerList? // null if customerList is null Text?.ToUpper() //from previous example, would return null This will make more sense with some examples below: Instead of having a crazy amount of “variable != null” type checks, you can use the “?” and your code will short circuit and return null instead of throwing the exception. One of the best new additions to C# was the null conditional operator. Exception! Object reference not set to an instance of an objectĬommand.ExecuteNonQuery() Use the Null Conditional Operator to Avoid NullReferenceExceptions Other times, like with the SqlCommand, it could be a fatal issue you don’t want to ignore.
A null string might be something you just ignore and move on. Not running a SQL query would be a serious problem for your application. For example, in the code below, the SqlCommand object is never initialized. You can also have null reference exceptions because any type of object is null. You can’t call ToUpper() on a null string. The following code will throw a NullReferenceException if the variable “text” being passed in is null. Here are some ways to avoid NullReferenceException. They are usually very simple problems caused by not adding additional logic to ensure that objects have valid values before using them. Null reference errors are responsible for a good percentage of all application bugs. > Understanding the NullReferenceException When you try to call a method or another member on the said variable, you got the NullReferenceException. These variables can point to “nothing”, though, and that’s what we call a null reference: a reference that doesn’t point to any object. So, a reference is what a variable of a reference type contains. Types such as int (and the other numerical primitive types), DateTime, and boolean are value types. If it helps you visualize it better, you can think of a reference as a linking pointing to a web page, or a shortcut pointing to a file on your computer. They hold a reference that points to where the object lives in memory. Reference types variables, on the other hand, don’t hold the value itself. If you have a variable of a value type, it stores the value itself.
NET, you can divide data types in two categories: value types and reference types. But what is a null reference? In what way does it differ from a non-null reference? We already know that the NullReferenceException is caused by a null reference.
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