Q:

Crystal Reports - In C#, what is comparable to the Number type? 

In an application I am supporting, I am debugging an issue where an "Arithmetic operation caused an overflow" exception is occurring on an occasional basis when a value that is being returned from a database is presented in a report.   I am confident that the value being presented is coming from the database as a C# decimal. 
However, in our report, the display field has the Number type.   I read here, under conversion considerations that it is possible to get Overflow exceptions when cast to a smaller type that cannot handle the size of the value returned, such as a float or a double. 
Google has failed me on this one, sadly - searching for 'Crystal Reports Number datatype size' yielded nebulous results that don't tell me anything about the Crystal Reports number datatype. 
So, I pose the question here - how does the Crystal Reports Number relate to C# numeric types?   Or do they relate? 

A:

It seems Crystal Reports "Number" type is likely stored as a varchar or nvarchar, according to this related answer: Minimum & Maximum Values in Crystal Reports 2008 Column; I don't know if this is true in your case.  I agree that searching for CR data types is difficult, nowhere was I able to find proper documentation on how CR stores data or information about the precision and scale of numbers it stores. 
A decimal has a precision of 28-29 significant digits, with a .MaxValue constant of 79,228,162,514,264,337,593,543,950,335.  I imagine (hope!)  this is more than sufficient for your needs, but the problem might be that CR has more or less precision and even though some values aren't necessarily out of range, they are represented with too much precision for the destination to handle. 
For example, see this question about an Oracle to decimal problem.  Also an interesting case shows up in this question about subtracting 0.5 from a large decimal value. 
As you can see, the conversion between data types can be frustrating at times.  You may want to experiment with smaller data types before generating a report, such as double.