c# - Unexpected results for floating point equals -

question not why 0.1 + 0.9 not equals 1.0. different behaviour of equals.

can explain why examples below works differently.

float q = 0.1f; float w = 0.9f; float summ = q + w;  q + w == 1.0f; // false summ == 1.0f; // true 

why operator == works differently?

the problem due fact intermediate calculations being performed in higher precision, , rules when round float precision different in each case.

according the docs

by default, in code x86 architectures compiler uses coprocessor's 80-bit registers hold intermediate results of floating-point calculations.

... compiler performs rounding on variables of type float correct precision assignments , casts , when parameters passed function"

float summ = q + w assignment, , hence rounded nearest float, in case 1.

q + w == 1.0f neither cast, assignment or function call, result of addition still extended precision float, close, not equal, 1.