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Set readonly fields in a constructor local function c#

10

The following does not compile.

public class A

 private readonly int i;

 public A()
 
 void SetI()
 
 i = 10; 
 

 SetI();
 

It fails with this error:

CS0191 A readonly field cannot be assigned to (except in a constructor or a variable initializer)

Technically are we not in the constructor still, since the visibility of the local function is limited, so I'm wondering why this does not compile.

c# constructor compiler-errors readonly local-functions

share|improve this question

edited yesterday

Kris Harper

2,99152865

asked yesterday

iliasilias

1,20421729

• 
  
  
  

  
  6
  

  
  
  
  
  
  

  
  
  The compiler turns the SetI local function into a separate class-level method. Since this separate class-level method is not a constructor, you are not allowed to assign to readonly fields from it.
  
  – canton7
  yesterday
  

  
  
  
  


• 
  
  
  

  
  6
  

  
  
  
  
  
  

  
  
  @canton7 that is the answer. Why not write it as one?
  
  – Jamiec
  yesterday
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  If you could do this then the variable could no longer be called readonly. By definition, it can only be set once, during construction or with an initializer.
  
  – Andy G
  yesterday
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Probably the local method could be called using reflection, which might allow modification of the readonly field after construction, so that's why they forbid it..?
  
  – Thomas Hilbert
  yesterday
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  There are two obvious workarounds if you do want this, incidentally: either make SetI return the desired value instead of assigning it directly, or use an out parameter. Both involve still doing the actual assignments in the constructor itself, of course. Because local functions are documented to be private methods, just with different scope rules, this is somewhat consistent. It would not technically be possible for C# to support this without a (rather obnoxious) change in the runtime to establish a "readonly path" of callers -- not really worth it.
  
  – Jeroen Mostert
  yesterday
  
  

  
  
  
  

 | 
show 2 more comments

10

The following does not compile.

public class A

 private readonly int i;

 public A()
 
 void SetI()
 
 i = 10; 
 

 SetI();
 

It fails with this error:

CS0191 A readonly field cannot be assigned to (except in a constructor or a variable initializer)

Technically are we not in the constructor still, since the visibility of the local function is limited, so I'm wondering why this does not compile.

c# constructor compiler-errors readonly local-functions

share|improve this question

edited yesterday

Kris Harper

2,99152865

asked yesterday

iliasilias

1,20421729

• 
  
  
  

  
  6
  

  
  
  
  
  
  

  
  
  The compiler turns the SetI local function into a separate class-level method. Since this separate class-level method is not a constructor, you are not allowed to assign to readonly fields from it.
  
  – canton7
  yesterday
  

  
  
  
  


• 
  
  
  

  
  6
  

  
  
  
  
  
  

  
  
  @canton7 that is the answer. Why not write it as one?
  
  – Jamiec
  yesterday
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  If you could do this then the variable could no longer be called readonly. By definition, it can only be set once, during construction or with an initializer.
  
  – Andy G
  yesterday
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Probably the local method could be called using reflection, which might allow modification of the readonly field after construction, so that's why they forbid it..?
  
  – Thomas Hilbert
  yesterday
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  There are two obvious workarounds if you do want this, incidentally: either make SetI return the desired value instead of assigning it directly, or use an out parameter. Both involve still doing the actual assignments in the constructor itself, of course. Because local functions are documented to be private methods, just with different scope rules, this is somewhat consistent. It would not technically be possible for C# to support this without a (rather obnoxious) change in the runtime to establish a "readonly path" of callers -- not really worth it.
  
  – Jeroen Mostert
  yesterday
  
  

  
  
  
  

 | 
show 2 more comments

The following does not compile.

public class A

 private readonly int i;

 public A()
 
 void SetI()
 
 i = 10; 
 

 SetI();
 

It fails with this error:

CS0191 A readonly field cannot be assigned to (except in a constructor or a variable initializer)

Technically are we not in the constructor still, since the visibility of the local function is limited, so I'm wondering why this does not compile.

c# constructor compiler-errors readonly local-functions

share|improve this question

edited yesterday

Kris Harper

2,99152865

asked yesterday

iliasilias

1,20421729

public class A

 private readonly int i;

 public A()
 
 void SetI()
 
 i = 10; 
 

 SetI();
 

share|improve this question

edited yesterday

Kris Harper

2,99152865

asked yesterday

iliasilias

1,20421729

share|improve this question

edited yesterday

Kris Harper

2,99152865

asked yesterday

iliasilias

1,20421729

edited yesterday

Kris Harper

2,99152865

edited yesterday

Kris Harper

2,99152865

asked yesterday

iliasilias

1,20421729

asked yesterday

iliasilias

1,20421729

1 Answer
1

active

oldest

votes

16

The compiler turns the SetI local function into a separate class-level method. Since this separate class-level method is not a constructor, you are not allowed to assign to readonly fields from it.

So the compiler takes this:

public class A

 private readonly int i;

 public A()
 
 void SetI()
 
 i = 10; 
 

 SetI();
 

and turns it into this:

public class A

 private readonly int i;

 public A()
 
 <.ctor>g__SetI

 [CompilerGenerated]
 private void <.ctor>g__SetI

(SharpLab. I left off the readonly so it would compile.)

As you can see, it's trying to assign i from the method <.ctor>g__SetI|1_0(), which isn't a constructor.

Unfortunately the C# 7.0 language specification hasn't yet been published, so I can't quote it.

Exactly the same happens if you try and use a delegate:

public class A

 private readonly int i;

 public A()
 
 Action setI = () => i = 10;

 setI();
 

Gets compiled to:

public class A

 private readonly int i;

 public A()
 
 Action action = <.ctor>b__1_0;
 action();
 

 [CompilerGenerated]
 private void <.ctor>b__1_0()
 
 i = 10;
 

(SharpLab, again with the readonly.)

... which likewise fails to compile.

share|improve this answer

edited yesterday

answered yesterday

canton7canton7

4,3761728

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks @canton7, I am looking forward to the c#7/8 language specification when/if it arrives.
  
  – ilias
  yesterday
  
  

  
  
  
  

add a comment | 

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16

The compiler turns the SetI local function into a separate class-level method. Since this separate class-level method is not a constructor, you are not allowed to assign to readonly fields from it.

So the compiler takes this:

public class A

 private readonly int i;

 public A()
 
 void SetI()
 
 i = 10; 
 

 SetI();
 

and turns it into this:

public class A

 private readonly int i;

 public A()
 
 <.ctor>g__SetI

 [CompilerGenerated]
 private void <.ctor>g__SetI

(SharpLab. I left off the readonly so it would compile.)

As you can see, it's trying to assign i from the method <.ctor>g__SetI|1_0(), which isn't a constructor.

Unfortunately the C# 7.0 language specification hasn't yet been published, so I can't quote it.

Exactly the same happens if you try and use a delegate:

public class A

 private readonly int i;

 public A()
 
 Action setI = () => i = 10;

 setI();
 

Gets compiled to:

public class A

 private readonly int i;

 public A()
 
 Action action = <.ctor>b__1_0;
 action();
 

 [CompilerGenerated]
 private void <.ctor>b__1_0()
 
 i = 10;
 

(SharpLab, again with the readonly.)

... which likewise fails to compile.

share|improve this answer

edited yesterday

answered yesterday

canton7canton7

4,3761728

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks @canton7, I am looking forward to the c#7/8 language specification when/if it arrives.
  
  – ilias
  yesterday
  
  

  
  
  
  

add a comment | 

16

The compiler turns the SetI local function into a separate class-level method. Since this separate class-level method is not a constructor, you are not allowed to assign to readonly fields from it.

So the compiler takes this:

public class A

 private readonly int i;

 public A()
 
 void SetI()
 
 i = 10; 
 

 SetI();
 

and turns it into this:

public class A

 private readonly int i;

 public A()
 
 <.ctor>g__SetI

 [CompilerGenerated]
 private void <.ctor>g__SetI

(SharpLab. I left off the readonly so it would compile.)

As you can see, it's trying to assign i from the method <.ctor>g__SetI|1_0(), which isn't a constructor.

Unfortunately the C# 7.0 language specification hasn't yet been published, so I can't quote it.

Exactly the same happens if you try and use a delegate:

public class A

 private readonly int i;

 public A()
 
 Action setI = () => i = 10;

 setI();
 

Gets compiled to:

public class A

 private readonly int i;

 public A()
 
 Action action = <.ctor>b__1_0;
 action();
 

 [CompilerGenerated]
 private void <.ctor>b__1_0()
 
 i = 10;
 

(SharpLab, again with the readonly.)

... which likewise fails to compile.

share|improve this answer

edited yesterday

answered yesterday

canton7canton7

4,3761728

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks @canton7, I am looking forward to the c#7/8 language specification when/if it arrives.
  
  – ilias
  yesterday
  
  

  
  
  
  

add a comment | 

The compiler turns the SetI local function into a separate class-level method. Since this separate class-level method is not a constructor, you are not allowed to assign to readonly fields from it.

So the compiler takes this:

public class A

 private readonly int i;

 public A()
 
 void SetI()
 
 i = 10; 
 

 SetI();
 

and turns it into this:

public class A

 private readonly int i;

 public A()
 
 <.ctor>g__SetI

 [CompilerGenerated]
 private void <.ctor>g__SetI

(SharpLab. I left off the readonly so it would compile.)

As you can see, it's trying to assign i from the method <.ctor>g__SetI|1_0(), which isn't a constructor.

Unfortunately the C# 7.0 language specification hasn't yet been published, so I can't quote it.

Exactly the same happens if you try and use a delegate:

public class A

 private readonly int i;

 public A()
 
 Action setI = () => i = 10;

 setI();
 

Gets compiled to:

public class A

 private readonly int i;

 public A()
 
 Action action = <.ctor>b__1_0;
 action();
 

 [CompilerGenerated]
 private void <.ctor>b__1_0()
 
 i = 10;
 

(SharpLab, again with the readonly.)

... which likewise fails to compile.

share|improve this answer

edited yesterday

answered yesterday

canton7canton7

4,3761728

public class A

 private readonly int i;

 public A()
 
 void SetI()
 
 i = 10; 
 

 SetI();
 

public class A

 private readonly int i;

 public A()
 
 <.ctor>g__SetI

 [CompilerGenerated]
 private void <.ctor>g__SetI

public class A

 private readonly int i;

 public A()
 
 Action setI = () => i = 10;

 setI();
 

public class A

 private readonly int i;

 public A()
 
 Action action = <.ctor>b__1_0;
 action();
 

 [CompilerGenerated]
 private void <.ctor>b__1_0()
 
 i = 10;
 

share|improve this answer

edited yesterday

answered yesterday

canton7canton7

4,3761728

answered yesterday

canton7canton7

4,3761728

answered yesterday

canton7canton7

4,3761728