Widening and Narrowing Conversions

int is of 4 bytes, and, double is of 8 bytes in c#

Casting means cropping some bits of a variable to fit a it's value into a smaller sized variable's bucket of bits and hence stripping off some value to fit the type of the target variable (Applicable for primitive type variables)

So, in this case, if we have the following two variables:

int a;
double d = 2.567;

we would need to cast the double variable d to an int if we want to assign it to the int variable a.

a = (int)d; // a would have value assigned to 2 in this case.

So, casting is a "narrowing conversion" here as the binary value representations of 8 bytes cannot fit into a 4 byte variable.

However, if we need to assign an int value to a double variable as follows, we wouldn't require any casting:

int a = 5;
double d;

d = a; // d would have value assigned to 5.0

This is a "widening conversation" as the binary value representations of 4 bytes are being assigned to a 8 byte variable.