When you have a statement like A.B.C = E.F;, and you receive an NullReferenceException 'Object reference not set to an instance of an object.' on that line of code, it basically means either A, or B or E are null.
To figure out what's wrong there, you can set a breakpoint on the same line, start debugging and then when the breakpoint hits, just check the value of A, B or E.
When NullReferenceException happens?
There are some common cases that you may encounter a NullReferenceException. Here is a summary based on NllReferenceEexception article in .NET documentations. I've put the link of the article in the next section in the answer, where you can find some useful example as well.
A NullReferenceException exception is thrown when you try to access a member on a type whose value is null. A NullReferenceException exception typically reflects developer error and is thrown in the following scenarios:
A
Object -> Exception -> SystemException -> IOException -> FileNotFoundException1 occurs when an invalid argument is passed to a method in C#. In this case, it refers to the passing of a null object when the method expects a non-null object or a value. Similar to other exceptions raised as a result of arguments,
Object -> Exception -> SystemException -> IOException -> FileNotFoundException1 is not generally raised by the
Object -> Exception -> SystemException -> IOException -> FileNotFoundException3 framework itself or the Common Language Runtime [CLR]. Instead, it is thrown by an application or a library as an indication of improper null arguments.
Syntax of ArgumentNullException
Similar to any class or method, exceptions also have their own syntax.
Below is the syntax for ArgumentNullException:
public class ArgumentNullException : ArgumentExceptionThe
Object -> Exception -> SystemException -> IOException -> FileNotFoundException4comes under the class of
Object -> Exception -> SystemException -> IOException -> FileNotFoundException5, which is inherited from the
Object -> Exception -> SystemException -> IOException -> FileNotFoundException6 class. The
Object -> Exception -> SystemException -> IOException -> FileNotFoundException6 class is in turn inherited from the
Object -> Exception -> SystemException -> IOException -> FileNotFoundException8 class, which is inherited from the
Object -> Exception -> SystemException -> IOException -> FileNotFoundException9 class.
Object -> Exception -> SystemException -> IOException -> FileNotFoundExceptionWhen does the ArgumentNullException occur in C#?
Generally, there are two major circumstances when an
class Program
{
static void Main[string[] args]
{
string id = null;
int ans = int.Parse[id]; // error is thrown
}
}
0 is thrown, both of which reflect developer errors:
- An object returned from a method call is then passed as an argument to a second method, but the value of the original returned object is null. To prevent the error, check for a return value that is null and call the second method only if the return value is not null.
- An uninstantiated object is passed to a method. To prevent the error, instantiate the object.
Example One: Working with an Inbuilt Function like Parse[]
In the below code, we are trying to parse and convert a string value to an integer value, assuming that the string is valid and contains only numbers.
class Program
{
static void Main[string[] args]
{
string id = null;
int ans = int.Parse[id]; // error is thrown
}
}
Output of Example 1
We can see that
class Program
{
static void Main[string[] args]
{
string id = null;
int ans = int.Parse[id]; // error is thrown
}
}
1 is causing the error because its parameter should not be null.
Unhandled Exception: System.ArgumentNullException: Value cannot be null.
Parameter name: String
at System.Number.StringToNumber[String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal]
at System.Number.ParseInt32[String s, NumberStyles style, NumberFormatInfo info]
at System.Int32.Parse[String s]
at ConsoleApp1.Program.Main[String[] args] in C:\ConsoleApp1\ConsoleApp1\Program.cs:line 50Example Two: Dealing with Custom Classes
In the below code we have created a class
class Program
{
static void Main[string[] args]
{
string id = null;
int ans = int.Parse[id]; // error is thrown
}
}
2, two private strings,
class Program
{
static void Main[string[] args]
{
string id = null;
int ans = int.Parse[id]; // error is thrown
}
}
3 and
class Program
{
static void Main[string[] args]
{
string id = null;
int ans = int.Parse[id]; // error is thrown
}
}
4, and used them for the public properties of
class Program
{
static void Main[string[] args]
{
string id = null;
int ans = int.Parse[id]; // error is thrown
}
}
5and
class Program
{
static void Main[string[] args]
{
string id = null;
int ans = int.Parse[id]; // error is thrown
}
}
6respectively. Then we wrote custom
class Program
{
static void Main[string[] args]
{
string id = null;
int ans = int.Parse[id]; // error is thrown
}
}
7 and
class Program
{
static void Main[string[] args]
{
string id = null;
int ans = int.Parse[id]; // error is thrown
}
}
8 functions that checked whether or not the passed argument value was null.
If true, we throw in a new
Object -> Exception -> SystemException -> IOException -> FileNotFoundException1 instead of passing the entire message, as is frequently the case,
Object -> Exception -> SystemException -> IOException -> FileNotFoundException1 expects just the name of the argument, which should not be null.
namespace ConsoleApp1
{
public class Books
{
private string authors;
private string titles;
public string Author
{
get { return authors; }
set {
if [value is null]
throw new System.ArgumentNullException["Author"];
authors = value;
}
}
public string Title
{
get { return titles; }
set {
if [value is null]
throw new System.ArgumentNullException["Title"];
titles = value; }
}
public Books[string title, string author]
{
Author = author;
Title = title;
}
}
class Program
{
static void Main[string[] args]
{
var obj = new Books["Harry potter", null];
}
}
}Output of Example Two
When the above code is run we get the following output:
Unhandled Exception: System.ArgumentNullException: Value cannot be null.
Parameter name: Author
at ConsoleApp1.Books.set_Author[String value] in C:\ConsoleApp1\ConsoleApp1\Program.cs:line 21
at ConsoleApp1.Books..ctor[String title, String author] in C:\ConsoleApp1\ConsoleApp1\Program.cs:line 39
at ConsoleApp1.Program.Main[String[] args] in C:\ConsoleApp1\ConsoleApp1\Program.cs:line 49The parameter
Unhandled Exception: System.ArgumentNullException: Value cannot be null.
Parameter name: String
at System.Number.StringToNumber[String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal]
at System.Number.ParseInt32[String s, NumberStyles style, NumberFormatInfo info]
at System.Int32.Parse[String s]
at ConsoleApp1.Program.Main[String[] args] in C:\ConsoleApp1\ConsoleApp1\Program.cs:line 501 is causing this exception as its value should not be null.
How to Handle ArgumentNullException in C#
Now let’s see how to debug and handle this exception in C#. The best approach is to use
Unhandled Exception: System.ArgumentNullException: Value cannot be null.
Parameter name: String
at System.Number.StringToNumber[String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal]
at System.Number.ParseInt32[String s, NumberStyles style, NumberFormatInfo info]
at System.Int32.Parse[String s]
at ConsoleApp1.Program.Main[String[] args] in C:\ConsoleApp1\ConsoleApp1\Program.cs:line 502 block and perform a simple check before passing the values. Let’s see how to fix both examples discussed earlier.
How to Fix Example One:
On observing the first few lines of the output from the bottom to the top, it is quite evident that when parsing a string to convert the string to a number,
Object -> Exception -> SystemException -> IOException -> FileNotFoundException1 occurs as the argument of
Unhandled Exception: System.ArgumentNullException: Value cannot be null.
Parameter name: String
at System.Number.StringToNumber[String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal]
at System.Number.ParseInt32[String s, NumberStyles style, NumberFormatInfo info]
at System.Int32.Parse[String s]
at ConsoleApp1.Program.Main[String[] args] in C:\ConsoleApp1\ConsoleApp1\Program.cs:line 504cannot be null.
Unhandled Exception: System.ArgumentNullException: Value cannot be null.
Parameter name: String
at System.Number.StringToNumber[String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal]
at System.Number.ParseInt32[String s, NumberStyles style, NumberFormatInfo info]Working Code:
class Program
{
static void Main[string[] args]
{
try
{
string id = null;
if [id is null]
{
throw new ArgumentNullException["Id Argument cannot be null"];
}
else {
int ans = int.Parse[id];
}
}catch [ArgumentNullException e]
{
Console.WriteLine[e.Message];
}
}
}
Output:
Value cannot be null.
Parameter name: Id Argument cannot be nullHow to Fix Example Two:
Implement a
Unhandled Exception: System.ArgumentNullException: Value cannot be null.
Parameter name: String
at System.Number.StringToNumber[String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal]
at System.Number.ParseInt32[String s, NumberStyles style, NumberFormatInfo info]
at System.Int32.Parse[String s]
at ConsoleApp1.Program.Main[String[] args] in C:\ConsoleApp1\ConsoleApp1\Program.cs:line 502 block in this case because the value is being checked in the
Unhandled Exception: System.ArgumentNullException: Value cannot be null.
Parameter name: String
at System.Number.StringToNumber[String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal]
at System.Number.ParseInt32[String s, NumberStyles style, NumberFormatInfo info]
at System.Int32.Parse[String s]
at ConsoleApp1.Program.Main[String[] args] in C:\ConsoleApp1\ConsoleApp1\Program.cs:line 506method.
Working Code:
namespace ConsoleApp1
{
public class Books
{
private string authors;
private string titles;
public string Author
{
get { return authors; }
set {
if [value is null]
throw new System.ArgumentNullException["Author"];
authors = value; }
}
public string Title
{
get { return titles; }
set {
if [value is null]
throw new System.ArgumentNullException["Title"];
titles = value; }
}
public Books[string title, string author]
{
Author = author;
Title = title;
}
}
class Program
{
static void Main[string[] args]
{
try
{
var obj = new Books["Harry potter", null];
}catch[ArgumentNullException e]
{
Console.WriteLine[e.Message];
}
}
}
}
Output:
Object -> Exception -> SystemException -> IOException -> FileNotFoundException0
Avoiding ArgumentNullExceptions
To summarize, an
class Program
{
static void Main[string[] args]
{
string id = null;
int ans = int.Parse[id]; // error is thrown
}
}
0 comes from
Unhandled Exception: System.ArgumentNullException: Value cannot be null.
Parameter name: String
at System.Number.StringToNumber[String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal]
at System.Number.ParseInt32[String s, NumberStyles style, NumberFormatInfo info]
at System.Int32.Parse[String s]
at ConsoleApp1.Program.Main[String[] args] in C:\ConsoleApp1\ConsoleApp1\Program.cs:line 508 when an invalid argument is passed to a method. In this case, it refers to passing a null object when the method expects a non-null object or a value. Furthermore, whenever dealing with
Unhandled Exception: System.ArgumentNullException: Value cannot be null.
Parameter name: String
at System.Number.StringToNumber[String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal]
at System.Number.ParseInt32[String s, NumberStyles style, NumberFormatInfo info]
at System.Int32.Parse[String s]
at ConsoleApp1.Program.Main[String[] args] in C:\ConsoleApp1\ConsoleApp1\Program.cs:line 509, it’s always good practice to perform a
namespace ConsoleApp1
{
public class Books
{
private string authors;
private string titles;
public string Author
{
get { return authors; }
set {
if [value is null]
throw new System.ArgumentNullException["Author"];
authors = value;
}
}
public string Title
{
get { return titles; }
set {
if [value is null]
throw new System.ArgumentNullException["Title"];
titles = value; }
}
public Books[string title, string author]
{
Author = author;
Title = title;
}
}
class Program
{
static void Main[string[] args]
{
var obj = new Books["Harry potter", null];
}
}
}0 and then pass any arguments.
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