Variables and passing arguments

Forms that compile (15)

Syntax	Description	Note
int a = 1;	Local variable
var a = 1;	Type inference
int a = 1, b = 2;	Combined declaration
{ int a = 1; }	Scope from a block
int a; a = 1;	Assign after declaring	Reading it before assignment is a CS0165 error.
private static int Counter;	Static variable	The value is separate for each instance of the Behaviour and is not shared between instances.
int Take(int v)	Pass by value	Assigning to the parameter in the callee doesn’t change the caller’s variable.
int Read(in int v)	in parameter
bool TryHalf(int v, out int result)	out parameters	What you may pass is a local variable or a parameter of the calling method, or a field the callee itself does not read or write. An array element cannot be passed (there is no way to tell whether the same array is also reachable under another name).
void Bump(ref int v)	ref parameters	The same restriction on what may be passed applies as for out.
int.TryParse("1", out n)	out from a method on the runtime
out int n	Declare an out variable inline
if (…) a = 1; else a = 2;	Assignment in if and else
switch … case … default …	Assignment in every switch case
_ = Make();	Discard a return value

Forms that don’t compile (7)

Syntax	Description	Error	Reason	Alternative
Bump(ref a[0])	Pass an array element by ref	TUKI0001	by design	Take it into a local variable first, then assign that to the element
ref int r = ref a;	A local variable that refers to another variable (a ref local)	TUKI0001	undecided	Hold a value
ref int First(int[] xs)	Return the variable itself	TUKI0001	undecided	Return a value and write it back at the call site
r = ref b;	Replace what it points to	TUKI0001	undecided	Same as above
ref (c ? ref a : ref b)	Choose what it points to by a condition	TUKI0001	undecided	Same as above
Make(out _);	Discard the out destination	TUKI0001, TUKI0099	not yet	Receive it with a variable
int.TryParse("1", out _)	Discard the out of a method on the runtime	TUKI0099	not yet	Receive it with a variable

Forms that hold a variable’s location as a value	None of them are supported. ref locals, ref returns, ref reassignment, and ref in a conditional expression are all TUKI0001 (return the modified result and assign it at the call site).
Definite assignment	C#‘s rules apply as-is. Assigning in every branch lets you read it afterwards, but reading before assignment is CS0165

Declaration

Local variable

using UnityEngine;
using Tsukimi;

public class VarLocal : TsukimiBehaviour
{
    void Start()
    {
        int a = 1;
        Debug.Log(a);   // => 1
    }
}

Type inference

using UnityEngine;
using Tsukimi;

public class VarInferredType : TsukimiBehaviour
{
    void Start()
    {
        var a = 1;
        Debug.Log(a);   // => 1
    }
}

Combined declaration

using UnityEngine;
using Tsukimi;

public class VarMultipleDeclarators : TsukimiBehaviour
{
    void Start()
    {
        int a = 1, b = 2;
        Debug.Log(a + b);   // => 3
    }
}

Scope from a block

using UnityEngine;
using Tsukimi;

public class VarBlockScope : TsukimiBehaviour
{
    void Start()
    {
        {
            int a = 1;
            Debug.Log(a);   // => 1
        }
    }
}

Assign after declaring

It can be declared first and assigned later.

using UnityEngine;
using Tsukimi;

public class VarAssignBeforeUse : TsukimiBehaviour
{
    void Start()
    {
        int a;
        a = 1;
        Debug.Log(a);   // => 1
    }
}

Static variable

It belongs to the type rather than to an instance.

using UnityEngine;
using Tsukimi;

public class VaStatic : TsukimiBehaviour
{
    void Start()
    {
        Counter = Counter + 1;
        Debug.Log(Counter);   // => 1
    }

    private static int Counter;
}

Arguments

Pass by value

A copy of the value is passed to the parameter.

using UnityEngine;
using Tsukimi;

public class VaValueParam : TsukimiBehaviour
{
    void Start()
    {
        Debug.Log(Take(1));   // => 1
    }

    private int Take(int v) { return v; }
}

in parameter

That the callee won’t assign to the parameter can be stated in the declaration.

using UnityEngine;
using Tsukimi;

public class VarInParameter : TsukimiBehaviour
{
    private int Read(in int v) { return v; }

    void Start()
    {
        int a = 1;
        Debug.Log(Read(in a));   // => 1
    }
}

out parameters

The method you call writes the value.

using UnityEngine;
using Tsukimi;

public class VarOutParameter : TsukimiBehaviour
{
    private bool TryHalf(int v, out int result)
    {
        result = v / 2;
        return v % 2 == 0;
    }

    void Start()
    {
        int half;
        bool even = TryHalf(10, out half);
        Debug.Log(even);   // => true
        Debug.Log(half);   // => 5
    }
}

ref parameters

The method you call reads it and then changes it.

using UnityEngine;
using Tsukimi;

public class VarRefParameter : TsukimiBehaviour
{
    private void Bump(ref int v)
    {
        v = v + 1;
    }

    void Start()
    {
        int count = 1;
        Bump(ref count);
        Bump(ref count);
        Debug.Log(count);   // => 3
    }
}

out from a method on the runtime

using UnityEngine;
using Tsukimi;

public class VarOutFromExtern : TsukimiBehaviour
{
    void Start()
    {
        int n;
        bool ok = int.TryParse("1", out n);
        Debug.Log(ok ? n : 0);   // => 1
    }
}

Declare an out variable inline

using UnityEngine;
using Tsukimi;

public class VarOutDeclaredInline : TsukimiBehaviour
{
    void Start()
    {
        bool ok = int.TryParse("1", out int n);
        Debug.Log(ok ? n : 0);   // => 1
    }
}

Definite assignment

Assignment in if and else

using UnityEngine;
using Tsukimi;

public class VaDefAssignIf : TsukimiBehaviour
{
    void Start()
    {
        int a;
        if (true) { a = 1; } else { a = 2; }
        Debug.Log(a);   // => 1
    }
}

Assignment in every switch case

using UnityEngine;
using Tsukimi;

public class VaDefAssignSwitch : TsukimiBehaviour
{
    void Start()
    {
        int k = 1;
        int a;
        switch (k) { case 1: a = 1; break; default: a = 0; break; }
        Debug.Log(a);   // => 1
    }
}

Discard

Discard a return value

using UnityEngine;
using Tsukimi;

public class VarDiscard : TsukimiBehaviour
{
    private int calls;

    void Start()
    {
        _ = Make();
        Debug.Log(calls);   // => 1
    }

    private int Make()
    {
        calls = calls + 1;
        return 1;
    }
}