Which is better Constant [Comparison Operator] Variable Value or Variable Value [Comparison Operator] Constant? An introduction to Yoda Condition

Introduction

Many of us has the habit of writing the programming logic as follows

Case 1:

int a = 5;
if (a == 6){
    //do something
}

or Case 2:

bool x = false;
if (x == true){
    //do something
}

That means we are using equality comparison operator to compare the values.But, if by mistake we write like

Case 1:

int a = 5;
if (a = 6){
    //do something
}

or Case 2:

bool x = false;
if (x = true){
    //do something
}

Then the whole purpose of conditional checking will be wrong.

I do agree that for the first case, C# will give a compilation error Cannot implicitly convert type 'int' to 'bool'

while for the second case the it leaves away by giving an warning Assignment in conditional expression is always constant;did you mean to use == instead of = ?

The Yoda style

A better way to write code in that case will be in the Yoda style as below

Case 1:

int a = 5;
if (6 == a){
    //do something
}

or Case 2:

bool x = false;
if (true == x){
    //do something
}

Why?Because it reduces coding errors where we unintentionally mistyped the comparison operator == as a single =

Case 1:

int a = 5;
if (6 = a){
    //do something
}

or Case 2:

bool x = false;
if (true = x){
    //do something
}

C# will give compilation error in both the cases as The left-hand side of an assignment must be a variable, property or indexer

In simple words,we can't assign a value to a literal

Usage of this conditions style was popular on C/C++ where we could assign value to variable when typed = instead of ==

Does it really needed in C#?

Well from all the above experiments (atleast Case 1), we have seen that in C# expressions are not converted automatically to bool and henceforth this practice does not make much sence in C# parlance

However,the convention should be to always put the constant on the left hand side of the expression so that an error can be statically detected at compile time in the event of a typo e.g. = (assignment) was coded instead of the == (equality).

How IL interpretes?

To answer this question, we have developed a simple program as under

public bool ConstantLeft(int a)
     {
        return a == 1;
     }

    public bool ConstantRight(int a)
    {
        return 1 == a;
    }

When viewed in IL, we found the below for ConstantLeft() function

.method public hidebysig instance bool  ConstantLeft(int32 a) cil managed
{
  // Code size       10 (0xa)
  .maxstack  2
  .locals init ([0] bool CS$1$0000)
  IL_0000:  nop
  IL_0001:  ldarg.1
  IL_0002:  ldc.i4.1
  IL_0003:  ceq
  IL_0005:  stloc.0
  IL_0006:  br.s       IL_0008
  IL_0008:  ldloc.0
  IL_0009:  ret
} // end of method Class1::ConstantLeft

For ConstantRight() function, IL view is as under

.method public hidebysig instance bool  ConstantRight(int32 a) cil managed
{
  // Code size       10 (0xa)
  .maxstack  2
  .locals init ([0] bool CS$1$0000)
  IL_0000:  nop
  IL_0001:  ldc.i4.1
  IL_0002:  ldarg.1
  IL_0003:  ceq
  IL_0005:  stloc.0
  IL_0006:  br.s       IL_0008
  IL_0008:  ldloc.0
  IL_0009:  ret
} // end of method Class1::ConstantRight

The IL infers that, line numbers IL_0001 and IL_0002, they're just swapped but the order of the operands doesn't change the behavior of ceq at IL_0003.So apart from swapping, there was no change found.

Reference

Conclusion

Hope this little tip will be helpful. Comments and suggestions are welcome.

Thanks for reading.