Nullable Types are introduced in C#2.0, they allow you to assign null to a value type variable. Before going through in details, it is important to know Reference types and Value Types.
What is Nullable Type in C#?
In C#, you cannot assign null to a value type variable, if you write something like this i.e int x = null, then this will give compile time error.
However, you can declare a value type as Nullable Type and then assign null value. You can declare nullable type using Nullable<T> where T is a type, for example.
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Nullable<Int32> i = null; //Declared a null-able type of System.Int32 Nullable<DateTime> dtJoin = null; //Declared a null-able type of System.DateTime Nullable<Boolean> bStatus = null; //Declared a null-able type of System.Boolean Nullable<Decimal> Salary = null; //Declared a null-able type of System.Decimal |
We can also declare nullable type using question mark (?) operator as follows.
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Int32? i = null; //Declared a null-able type of System.Int32 DateTime? dtjoin = null; //Declared a null-able type of System.DateTime Boolean? bStatus = null; //Declared a null-able type of System.Boolean Decimal? Salary = null; //Declared a null-able type of System.Decimal |
Example : Here in below example, we have declared a variable as int i = 0 . Now if you write int i = null then compiler throws error as shown below.
We can declare above value type as Nullable and assign null value at run time.
In below example code, we have declared a Nullable of type int and assigned null value at run-time.
Nullable types are mostly applicable to value types, you can’t declare a reference type as Nullable , because by default reference types supports null value.
In below example System.String is a reference type so compiler throws error.
Conversion of Nullable Type in C#:
C# allows to perform conversions and casts on Nullable types. Here in below, implicit conversion from non-nullable Int32 to Nullable<Int32> .
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Int32? i = 100; //Implicit conversion from non-nullable Int32 to Nullable<Int32> |
Similarly in below example, explicit conversion Nullable<Int32> to non-nullable Int32 .
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Int32 j = (Int32)i //Explicit conversion from Nullable<Int32> to non-nullable Int32 |
HasValue() property of Nullable Type :
The Nullable types are instances of System.Nullable<T> struct. It has a property as HasValue . It returns True, if the variable contains a value and returns False if it’s null.
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class Program { static void Main(string[] args) { Nullable i = null; //Nullable declaration int? j = null; //Nullable declaration i = 123;//Assign a value if (i.HasValue)//Check if i has value in it Console.WriteLine("i containes value as - " + i); else Console.WriteLine("i containes null."); if (j.HasValue)//Check if j has value in it Console.WriteLine("j containes value as - " + j); else Console.WriteLine("j containes null."); } } |
Output:
Where to use Nullable Type?
Let’s say you have a boolean field in your database and that accept null value along with True/False, in that case you can define a Nullable of type bool and pass the null value as desired.
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bool? isActive = null; |
Important Points to be noted:
Below are some important points.
- Nullable types can only be used with value types.
- The HasValue() property of Nullable type returns True, if the variable contains a value and returns False if it’s null.
- Nullable<T> object returns type T instead of Nullable<T>. In below sample code, we have defined a nullable type and fetching the type information at runtime using GetType() method. When you run below program then, it will print System.Boolean instead of Nullable<Boolean>
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using System; namespace NullableType { class Program { static void Main(string[] args) { Nullable<Boolean> bStatus = false; Console.WriteLine(bStatus.GetType()); //Displays System.Boolean } } } |
Thanks for your time 🙂 feel free to provide your feedback and do check my other blogs on Dynamic type , Var type , Anonymous type and Reference Type and Value type .