var t = (decimal)((float)decimal.MinValue)
fails with error CS0031: Constant value '-7.922816E+28' cannot be converted to a 'decimal', but why?
decimal.MinValue is -79228162514264337593543950335m. It successfully converts to float as -79228160000000000000000000000f wichh is larger than decimal.MinValue:
(float)decimal.MinValue > decimal.MinValue
Why this value can't be converted? I understand it's something about the significand and the exponent but what exactly happen? And which is minimum decimal which can be converted to float and then back to decimal in C#?
You are not printing the full precision of the float, so you cannot see the value that is causing it to fail.
If you change the code as follows:
var decMin = (float)decimal.MinValue;
Console.WriteLine(decMin.ToString("R")); // "-7.9228163E+28"
// Next line throws System.OverflowException: Value was either too large or too small for a Decimal.
var dec = (decimal)decMin;
The value of decMin is displayed as -7.9228163E+28. To make this easier to compare, let's rewrite that without the exponent and compare it to decimal.MinValue:
decMin = -79_228_163_000_000_000_000_000_000_000
decimal.MinValue = -79_228_162_514_264_337_593_543_950_335
Now you can see that decMin is MORE NEGATIVE than decimal.MinValue - which explains why it fails to convert.