Question

I have a property that returns System.Numerics.BigInteger. When I casting the property to int, I got this error.

Cannot convert type 'System.Numerics.BigInteger' to 'int'

How can I convert int to/from System.Numerics.BigInteger in C#?

Answer 1

The conversion from BigInteger to Int32 is explicit, so just assigning a BigInteger variable/property to an int variable doesn’t work:

BigInteger big = ...

int result = big;           // compiler error:
                            //   "Cannot implicitly convert type
                            //    'System.Numerics.BigInteger' to 'int'.
                            //    An explicit conversion exists (are you
                            //    missing a cast?)"

This works (although it might throw an exception at runtime if the value is too large to fit in the int variable):

BigInteger big = ...

int result = (int)big;      // works

---

Note that, if the BigInteger value is boxed in an object, you cannot unbox it and convert it to int at the same time:

BigInteger original = ...;

object obj = original;      // box value

int result = (int)obj;      // runtime error
                            //   "Specified cast is not valid."

This works:

BigInteger original = ...;

object obj = original;            // box value

BigInteger big = (BigInteger)obj; // unbox value

int result = (int)big;            // works