Difference with method parameters ref and out
suggest changeThere are two possible ways to pass a value type by reference: ref
and out
. The difference is that by passing it with ref
the value must be initialized but not when passing it with out
. Using out
ensures that the variable has a value after the method call:
public void ByRef(ref int value)
{
Console.WriteLine(nameof(ByRef) + value);
value += 4;
Console.WriteLine(nameof(ByRef) + value);
}
public void ByOut(out int value)
{
value += 4 // CS0269: Use of unassigned out parameter `value'
Console.WriteLine(nameof(ByOut) + value); // CS0269: Use of unassigned out parameter `value'
value = 4;
Console.WriteLine(nameof(ByOut) + value);
}
public void TestOut()
{
int outValue1;
ByOut(out outValue1); // prints 4
int outValue2 = 10; // does not make any sense for out
ByOut(out outValue2); // prints 4
}
public void TestRef()
{
int refValue1;
ByRef(ref refValue1); // S0165 Use of unassigned local variable 'refValue'
int refValue2 = 0;
ByRef(ref refValue2); // prints 0 and 4
int refValue3 = 10;
ByRef(ref refValue3); // prints 10 and 14
}
The catch is that by using out
the parameter must
be initialized before leaving the method, therefore the following method is possible with ref
but not with out
:
public void EmtyRef(bool condition, ref int value)
{
if (condition)
{
value += 10;
}
}
public void EmtyOut(bool condition, out int value)
{
if (condition)
{
value = 10;
}
} //CS0177: The out parameter 'value' must be assigned before control leaves the current method
This is because if condition
does not hold, value
goes unassigned.
Found a mistake? Have a question or improvement idea?
Let me know.
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