Question

Given two shorts (System.Int16)

short left = short.MaxValue; short right = 1; 

I want to get an OverflowException when adding them.

checked(left+right) 

does not work, because the result of left+right is an Int32.

checked((short)(left+right)) 

works as expected.

My problem is that, using Expression Trees, the ‘trick’ doesn’t work:

var a = Expression.Constant(left); var b = Expression.Constant(right); var sum = Expression.ConvertChecked(Expression.Add(a, b), typeof(short)); var l = Expression.Lambda(sum); var f = (Func<short>)l.Compile(); 

Calling f() does not throw an overflow exception but returns -32768. What’s wrong?

Answer 1

The problem is that the addition is being done as short + short (which presumably exists in IL even if it doesn’t exist in C#) – and then the conversion is performed separately. This is shown by this complete program – even without the conversion, the result is -32768:

using System; using System.Linq.Expressions;  class Test {     static void Main(string[] args)     {         short left = short.MaxValue;         short right = 1;         var a = Expression.Constant(left);         var b = Expression.Constant(right);         var sum = Expression.Add(a, b);         var convert = Expression.ConvertChecked(sum, typeof(short));         var convertLambda = Expression.Lambda<Func<short>>(convert);         var convertFunc = convertLambda.Compile();         Console.WriteLine('Conversion: {0}', convertFunc());         var sumLambda = Expression.Lambda<Func<short>>(sum);         var sumFunc = sumLambda.Compile();         Console.WriteLine('Sum: {0}', sumFunc());     } } 

If you make it do an int + int addition and then the conversion, it will throw an overflow exception:

using System; using System.Linq.Expressions;  class Test {     static void Main(string[] args)     {         short left = short.MaxValue;         short right = 1;         var a = Expression.Constant((int) left);         var b = Expression.Constant((int) right);         var sum = Expression.Add(a, b);         var convert = Expression.ConvertChecked(sum, typeof(short));         var convertLambda = Expression.Lambda<Func<short>>(convert);         var convertFunc = convertLambda.Compile();         Console.WriteLine('Conversion: {0}', convertFunc());         var sumLambda = Expression.Lambda<Func<int>>(sum);         var sumFunc = sumLambda.Compile();         Console.WriteLine('Sum: {0}', sumFunc());     } } 

I don’t know why AddChecked doesn’t work though… that looks like a bug 🙁 It’s possible that using the overload which allows the Method to specified would work, but I’m not sure…