Invalid listing from 'System.String' to 'System.TimeSpan'

Question

I currently have a generic method that reads a value from a database based on a key and returns a specific type of that value.

public T Get<T>(string key, T defaultValue) { var myparms = new List<SqlParameter> { new SqlParameter("@KEY", key), }; const string getPropertyQuery = "SELECT SPARM_VALUE FROM SYSPARAMS WHERE SPARM_KEY = @KEY;"; var returnedValue = //Get value from Database if (returnedValue == null) { return defaultValue; //Value does not exists so return default. } return (T)Convert.ChangeType(returnedValue, typeof(T)); } 

But when I try to return a TimeSpan type, I get the following exception. Invalid cast from 'System.String' to 'System.TimeSpan'.

After several searches, I found that the most common solution is to use the TimeSpan.Parse or TimeSpan.TryParse methods.

I also found the TimeSpan Structure.

public struct TimeSpan : IComparable, IComparable, IEquatable, IFormattable

My question is why, why is TimeSpan unable to do this. Is it because it does not have an IConvertible interface? Any thoughts on this would be greatly appreciated.

Answer 1

I don't think Convert.ChangeType is what you really want. Try TypeConverter.ConvertFrom :

 var converter = TypeDescriptor.GetConverter(typeof(T)); return converter.CanConvertFrom(returnedValue) ? (T)converter.ConvertFrom(returnedValue) : defaultValue; 

Answer 2

There is no explicit conversion / conversion defined between string and TimeSpan because most strings are not TimeSpan s.

If the framework defines such a composition, do you expect them to exist on DateTime and all other value types?

Answer 3

There is an implicit conversion between String and TimeSpan , you need to explicitly convert using Parse or TryParse