Question

I have a string and a Type, and I want to return the string value converted to that Type.

public static object StringToType(string value, Type propertyType)
{
    return Convert.ChangeType(value, propertyType, CultureInfo.InvariantCulture);
}

This returns an object that I can use in a property set value call:

public static void SetBasicPropertyValueFromString(object target,
                                                   string propName,
                                                   string value)   
{
  PropertyInfo prop = target.GetType().GetProperty(propName);
  object converted = StringToType(value, prop.PropertyType);
  prop.SetValue(target, converted, null);
}

This works for most basic types, except nullables.

[TestMethod]
public void IntTest()
{ //working
    Assert.AreEqual(1, ValueHelper.StringToType("1", typeof (int)));
    Assert.AreEqual(123, ValueHelper.StringToType("123", typeof (int)));
}

[TestMethod]
public void NullableIntTest()
{ //not working
    Assert.AreEqual(1, ValueHelper.StringToType("1", typeof (int?)));
    Assert.AreEqual(123, ValueHelper.StringToType("123", typeof (int?)));
    Assert.AreEqual(null, ValueHelper.StringToType(null, typeof (int?)));
}

NullableIntTest fails on first line with:

System.InvalidCastException: Invalid cast from ‘System.String’ to ‘System.Nullable`1[[System.Int32, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]’.

I’m having difficulty detemining if the type is nullable and changing the behaiour of the StringToType method.

Behaviour I am after:

If string is null or empty, return null, else convert as per the non-nullable type.

Result

Like Kirill’s answer, only with one ChangeType call.

public static object StringToType(string value, Type propertyType)
{
    var underlyingType = Nullable.GetUnderlyingType(propertyType);
    if (underlyingType != null)
    {
        //an underlying nullable type, so the type is nullable
        //apply logic for null or empty test
        if (String.IsNullOrEmpty(value)) return null;
    }
    return Convert.ChangeType(value,
                              underlyingType ?? propertyType,
                              CultureInfo.InvariantCulture);
}

Answer 1

You cannot use Convert.ChangeType on nullable types cause it is not inherited from IConvertible. You should rewrite your method.

public static object StringToType(string value, Type propertyType)
{
   var underlyingType = Nullable.GetUnderlyingType(propertyType);
   if(underlyingType == null)
          return Convert.ChangeType(value, propertyType,  CultureInfo.InvariantCulture);
   return String.IsNullOrEmpty(value)
          ? null
          : Convert.ChangeType(value, underlyingType, CultureInfo.InvariantCulture);
}