Nullable Array and Why Do We Need Them

I see code like this and I understand it as making an array nullable but I dont understand why do we need it since arrays are ref types, so they are already nullable.

So, my question is why do we need this?

private readonly decimal? _amounts = new decimal?[_count];

edited Jan 2 at 17:05

Uwe Keim

27.7k32134216

asked Jan 2 at 17:02

cd491415

315111

2

It looks like the main confusion here is that it is not a "nullable array of decimals", but "a regular array of nullable decimals".

– Yeldar Kurmangaliyev
Jan 2 at 17:08

Thanks, that explains it

– cd491415
Jan 2 at 17:11

add a comment |

I see code like this and I understand it as making an array nullable but I dont understand why do we need it since arrays are ref types, so they are already nullable.

So, my question is why do we need this?

private readonly decimal? _amounts = new decimal?[_count];

edited Jan 2 at 17:05

Uwe Keim

27.7k32134216

asked Jan 2 at 17:02

cd491415

315111

2

It looks like the main confusion here is that it is not a "nullable array of decimals", but "a regular array of nullable decimals".

– Yeldar Kurmangaliyev
Jan 2 at 17:08

Thanks, that explains it

– cd491415
Jan 2 at 17:11

add a comment |

I see code like this and I understand it as making an array nullable but I dont understand why do we need it since arrays are ref types, so they are already nullable.

So, my question is why do we need this?

private readonly decimal? _amounts = new decimal?[_count];

edited Jan 2 at 17:05

Uwe Keim

27.7k32134216

asked Jan 2 at 17:02

cd491415

315111

I see code like this and I understand it as making an array nullable but I dont understand why do we need it since arrays are ref types, so they are already nullable.

So, my question is why do we need this?

private readonly decimal? _amounts = new decimal?[_count];

c# arrays nullable

edited Jan 2 at 17:05

Uwe Keim

27.7k32134216

asked Jan 2 at 17:02

cd491415

315111

edited Jan 2 at 17:05

Uwe Keim

27.7k32134216

asked Jan 2 at 17:02

cd491415

315111

edited Jan 2 at 17:05

Uwe Keim

27.7k32134216

edited Jan 2 at 17:05

Uwe Keim

27.7k32134216

edited Jan 2 at 17:05

Uwe Keim

27.7k32134216

asked Jan 2 at 17:02

cd491415

315111

asked Jan 2 at 17:02

cd491415

315111

asked Jan 2 at 17:02

cd491415

315111

2

It looks like the main confusion here is that it is not a "nullable array of decimals", but "a regular array of nullable decimals".

– Yeldar Kurmangaliyev
Jan 2 at 17:08

Thanks, that explains it

– cd491415
Jan 2 at 17:11

add a comment |

2

It looks like the main confusion here is that it is not a "nullable array of decimals", but "a regular array of nullable decimals".

– Yeldar Kurmangaliyev
Jan 2 at 17:08

Thanks, that explains it

– cd491415
Jan 2 at 17:11

It looks like the main confusion here is that it is not a "nullable array of decimals", but "a regular array of nullable decimals".

– Yeldar Kurmangaliyev
Jan 2 at 17:08

Thanks, that explains it

– cd491415
Jan 2 at 17:11

add a comment |

4 Answers
4

active

oldest

votes

It's worth to mention that from C# 8.0 you can have a nullable reference type:
https://docs.microsoft.com/en-us/dotnet/csharp/tutorials/nullable-reference-types
beacause with C# 8.0 reference types cannot be null by default (if this feature is enabled in the project).

but as others mentioned this:

private readonly decimal? _amounts = new decimal?[_count];

means the value-type elements in array can be null. decimal is value type and normally you can't assing null to it but if you have decimal? then you can.

With C# 8.0 and nullable reference types feature enabled you MUST have nullable reference type (stated explicitly) if you want to assing null to them. You can declare like this:

private decimal?? _amounts;

Now it means that both elements in array can be null and entire array (_amounts variable) can be null.

So in general the question mark after element value type ? and before -> SomeValueType? means that elements in array can be null. From C# 8.0 (with feature enabled in project) question mark ? after array type SomeArrayType -> SomeArrayType? means that you can assign null to variable that holds reference to the array.

edited Jan 2 at 17:53

answered Jan 2 at 17:43

BART

34119

Awesome explanation. Much appreciated BART!

– cd491415
Jan 2 at 17:57

add a comment |

Declaring it as decimal? means that the elements the array contains can be null or non-null.

Without making it nullable, the elements the array can store cannot be null.

In other words, decimal? reads as "an array of nullable decimals". The ? is referring to the elements the array can contain and not the array itself as all arrays are reference types.

edited Jan 2 at 17:16

answered Jan 2 at 17:04

Aomine

42.6k74677

add a comment |

The nullable specifier here refers not to the array itself, but to the contents of it. As you correctly say, the array in itself is a reference type, and there fore, always nulable (at least until C#8).

A few examples of the difference:

decimal nonNullable = new decimal[2];

nonNullable[0] = 1;             //OK, a non null item

nonNullable[1] = null;          //Compile error: array doesn't accepts nulls

nonNullable = null;             //OK, set the array reference to null



decimal? nullable = new decimal?[2];

nullable[0] = 1;                //OK, a non null item

nullable[1] = null;             //OK, a null item (actually, a Nullable<decimal> instance)

nullable = null;                //OK, set the array reference to null

edited Jan 2 at 17:16

Yeldar Kurmangaliyev

25.2k94068

answered Jan 2 at 17:11

Alejandro

4,39722239

Please note my question initiates array as new decimal?[count], not new decimal[count]

– cd491415
Jan 2 at 17:13

1

@cd491415 Yes, I noted that. I put both for the sake of comparison to show in which cases they have different behavior, both your specific case and a made up one.

– Alejandro
Jan 2 at 18:42

add a comment |

There's a difference between the array object itself and the elements of the array. When you use decimal? you are declaring an array whose elements are nullable decimal values. If you were to use decimal? (I'm not sure that's valid syntax, but just for explanation purposes, assume it is) you would be declaring a variable that may refer to an array or may not refer to anything. The later is the useless case you describe because, like you say (at least in older versions of C#) all array variables are reference variables that can already be set to null.

To clarify the difference, consider the code posted by Alejandro, and also this code:

decimal? reallyNullable = null;



Array.Sort(reallyNullable); // ArgumentNullException -- there is no array here

Array.Sort(nullable); // OK, assuming null can be compared to decimal.

The reason the last line is OK is because there is actually an array object referenced here. The only thing that's null are some of the values in it.

edited Jan 2 at 17:27

answered Jan 2 at 17:14

BlueMonkMN

18.8k656120

add a comment |

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4 Answers
4

active

oldest

votes

4 Answers
4

active

oldest

votes

but as others mentioned this:

private readonly decimal? _amounts = new decimal?[_count];

means the value-type elements in array can be null. decimal is value type and normally you can't assing null to it but if you have decimal? then you can.

With C# 8.0 and nullable reference types feature enabled you MUST have nullable reference type (stated explicitly) if you want to assing null to them. You can declare like this:

private decimal?? _amounts;

Now it means that both elements in array can be null and entire array (_amounts variable) can be null.

edited Jan 2 at 17:53

answered Jan 2 at 17:43

BART

34119

Awesome explanation. Much appreciated BART!

– cd491415
Jan 2 at 17:57

add a comment |

but as others mentioned this:

private readonly decimal? _amounts = new decimal?[_count];

means the value-type elements in array can be null. decimal is value type and normally you can't assing null to it but if you have decimal? then you can.

With C# 8.0 and nullable reference types feature enabled you MUST have nullable reference type (stated explicitly) if you want to assing null to them. You can declare like this:

private decimal?? _amounts;

Now it means that both elements in array can be null and entire array (_amounts variable) can be null.

edited Jan 2 at 17:53

answered Jan 2 at 17:43

BART

34119

Awesome explanation. Much appreciated BART!

– cd491415
Jan 2 at 17:57

add a comment |

but as others mentioned this:

private readonly decimal? _amounts = new decimal?[_count];

means the value-type elements in array can be null. decimal is value type and normally you can't assing null to it but if you have decimal? then you can.

With C# 8.0 and nullable reference types feature enabled you MUST have nullable reference type (stated explicitly) if you want to assing null to them. You can declare like this:

private decimal?? _amounts;

Now it means that both elements in array can be null and entire array (_amounts variable) can be null.

edited Jan 2 at 17:53

answered Jan 2 at 17:43

BART

34119

but as others mentioned this:

private readonly decimal? _amounts = new decimal?[_count];

means the value-type elements in array can be null. decimal is value type and normally you can't assing null to it but if you have decimal? then you can.

With C# 8.0 and nullable reference types feature enabled you MUST have nullable reference type (stated explicitly) if you want to assing null to them. You can declare like this:

private decimal?? _amounts;

Now it means that both elements in array can be null and entire array (_amounts variable) can be null.

edited Jan 2 at 17:53

answered Jan 2 at 17:43

BART

34119

edited Jan 2 at 17:53

answered Jan 2 at 17:43

BART

34119

answered Jan 2 at 17:43

BART

34119

answered Jan 2 at 17:43

BART

34119

Awesome explanation. Much appreciated BART!

– cd491415
Jan 2 at 17:57

add a comment |

Awesome explanation. Much appreciated BART!

– cd491415
Jan 2 at 17:57

Awesome explanation. Much appreciated BART!

– cd491415
Jan 2 at 17:57

add a comment |

Declaring it as decimal? means that the elements the array contains can be null or non-null.

Without making it nullable, the elements the array can store cannot be null.

In other words, decimal? reads as "an array of nullable decimals". The ? is referring to the elements the array can contain and not the array itself as all arrays are reference types.

edited Jan 2 at 17:16

answered Jan 2 at 17:04

Aomine

42.6k74677

add a comment |

Declaring it as decimal? means that the elements the array contains can be null or non-null.

Without making it nullable, the elements the array can store cannot be null.

In other words, decimal? reads as "an array of nullable decimals". The ? is referring to the elements the array can contain and not the array itself as all arrays are reference types.

edited Jan 2 at 17:16

answered Jan 2 at 17:04

Aomine

42.6k74677

add a comment |

Declaring it as decimal? means that the elements the array contains can be null or non-null.

Without making it nullable, the elements the array can store cannot be null.

In other words, decimal? reads as "an array of nullable decimals". The ? is referring to the elements the array can contain and not the array itself as all arrays are reference types.

edited Jan 2 at 17:16

answered Jan 2 at 17:04

Aomine

42.6k74677

Declaring it as decimal? means that the elements the array contains can be null or non-null.

Without making it nullable, the elements the array can store cannot be null.

In other words, decimal? reads as "an array of nullable decimals". The ? is referring to the elements the array can contain and not the array itself as all arrays are reference types.

edited Jan 2 at 17:16

answered Jan 2 at 17:04

Aomine

42.6k74677

edited Jan 2 at 17:16

answered Jan 2 at 17:04

Aomine

42.6k74677

answered Jan 2 at 17:04

Aomine

42.6k74677

answered Jan 2 at 17:04

Aomine

42.6k74677

add a comment |

A few examples of the difference:

decimal nonNullable = new decimal[2];

nonNullable[0] = 1;             //OK, a non null item

nonNullable[1] = null;          //Compile error: array doesn't accepts nulls

nonNullable = null;             //OK, set the array reference to null



decimal? nullable = new decimal?[2];

nullable[0] = 1;                //OK, a non null item

nullable[1] = null;             //OK, a null item (actually, a Nullable<decimal> instance)

nullable = null;                //OK, set the array reference to null

edited Jan 2 at 17:16

Yeldar Kurmangaliyev

25.2k94068

answered Jan 2 at 17:11

Alejandro

4,39722239

Please note my question initiates array as new decimal?[count], not new decimal[count]

– cd491415
Jan 2 at 17:13

1

@cd491415 Yes, I noted that. I put both for the sake of comparison to show in which cases they have different behavior, both your specific case and a made up one.

– Alejandro
Jan 2 at 18:42

add a comment |

A few examples of the difference:

decimal nonNullable = new decimal[2];

nonNullable[0] = 1;             //OK, a non null item

nonNullable[1] = null;          //Compile error: array doesn't accepts nulls

nonNullable = null;             //OK, set the array reference to null



decimal? nullable = new decimal?[2];

nullable[0] = 1;                //OK, a non null item

nullable[1] = null;             //OK, a null item (actually, a Nullable<decimal> instance)

nullable = null;                //OK, set the array reference to null

edited Jan 2 at 17:16

Yeldar Kurmangaliyev

25.2k94068

answered Jan 2 at 17:11

Alejandro

4,39722239

Please note my question initiates array as new decimal?[count], not new decimal[count]

– cd491415
Jan 2 at 17:13

1

@cd491415 Yes, I noted that. I put both for the sake of comparison to show in which cases they have different behavior, both your specific case and a made up one.

– Alejandro
Jan 2 at 18:42

add a comment |

A few examples of the difference:

decimal nonNullable = new decimal[2];

nonNullable[0] = 1;             //OK, a non null item

nonNullable[1] = null;          //Compile error: array doesn't accepts nulls

nonNullable = null;             //OK, set the array reference to null



decimal? nullable = new decimal?[2];

nullable[0] = 1;                //OK, a non null item

nullable[1] = null;             //OK, a null item (actually, a Nullable<decimal> instance)

nullable = null;                //OK, set the array reference to null

edited Jan 2 at 17:16

Yeldar Kurmangaliyev

25.2k94068

answered Jan 2 at 17:11

Alejandro

4,39722239

A few examples of the difference:

decimal nonNullable = new decimal[2];

nonNullable[0] = 1;             //OK, a non null item

nonNullable[1] = null;          //Compile error: array doesn't accepts nulls

nonNullable = null;             //OK, set the array reference to null



decimal? nullable = new decimal?[2];

nullable[0] = 1;                //OK, a non null item

nullable[1] = null;             //OK, a null item (actually, a Nullable<decimal> instance)

nullable = null;                //OK, set the array reference to null

edited Jan 2 at 17:16

Yeldar Kurmangaliyev

25.2k94068

answered Jan 2 at 17:11

Alejandro

4,39722239

edited Jan 2 at 17:16

Yeldar Kurmangaliyev

25.2k94068

edited Jan 2 at 17:16

Yeldar Kurmangaliyev

25.2k94068

edited Jan 2 at 17:16

Yeldar Kurmangaliyev

25.2k94068

answered Jan 2 at 17:11

Alejandro

4,39722239

answered Jan 2 at 17:11

Alejandro

4,39722239

answered Jan 2 at 17:11

Alejandro

4,39722239

Please note my question initiates array as new decimal?[count], not new decimal[count]

– cd491415
Jan 2 at 17:13

1

@cd491415 Yes, I noted that. I put both for the sake of comparison to show in which cases they have different behavior, both your specific case and a made up one.

– Alejandro
Jan 2 at 18:42

add a comment |

Please note my question initiates array as new decimal?[count], not new decimal[count]

– cd491415
Jan 2 at 17:13

1

@cd491415 Yes, I noted that. I put both for the sake of comparison to show in which cases they have different behavior, both your specific case and a made up one.

– Alejandro
Jan 2 at 18:42

Please note my question initiates array as new decimal?[count], not new decimal[count]

– cd491415
Jan 2 at 17:13

@cd491415 Yes, I noted that. I put both for the sake of comparison to show in which cases they have different behavior, both your specific case and a made up one.

– Alejandro
Jan 2 at 18:42

add a comment |

To clarify the difference, consider the code posted by Alejandro, and also this code:

decimal? reallyNullable = null;



Array.Sort(reallyNullable); // ArgumentNullException -- there is no array here

Array.Sort(nullable); // OK, assuming null can be compared to decimal.

The reason the last line is OK is because there is actually an array object referenced here. The only thing that's null are some of the values in it.

edited Jan 2 at 17:27

answered Jan 2 at 17:14

BlueMonkMN

18.8k656120

add a comment |

To clarify the difference, consider the code posted by Alejandro, and also this code:

decimal? reallyNullable = null;



Array.Sort(reallyNullable); // ArgumentNullException -- there is no array here

Array.Sort(nullable); // OK, assuming null can be compared to decimal.

The reason the last line is OK is because there is actually an array object referenced here. The only thing that's null are some of the values in it.

edited Jan 2 at 17:27

answered Jan 2 at 17:14

BlueMonkMN

18.8k656120

add a comment |

To clarify the difference, consider the code posted by Alejandro, and also this code:

decimal? reallyNullable = null;



Array.Sort(reallyNullable); // ArgumentNullException -- there is no array here

Array.Sort(nullable); // OK, assuming null can be compared to decimal.

The reason the last line is OK is because there is actually an array object referenced here. The only thing that's null are some of the values in it.

edited Jan 2 at 17:27

answered Jan 2 at 17:14

BlueMonkMN

18.8k656120

To clarify the difference, consider the code posted by Alejandro, and also this code:

decimal? reallyNullable = null;



Array.Sort(reallyNullable); // ArgumentNullException -- there is no array here

Array.Sort(nullable); // OK, assuming null can be compared to decimal.

The reason the last line is OK is because there is actually an array object referenced here. The only thing that's null are some of the values in it.

edited Jan 2 at 17:27

answered Jan 2 at 17:14

BlueMonkMN

18.8k656120

edited Jan 2 at 17:27

answered Jan 2 at 17:14

BlueMonkMN

18.8k656120

answered Jan 2 at 17:14

BlueMonkMN

18.8k656120

answered Jan 2 at 17:14

BlueMonkMN

18.8k656120

add a comment |

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