How to use variable type in as operator

Question

For example:I have 2 variable (value ) & (property) i want to check is cast possible for value? We do not know the type of variables, How to check if cast is possible?

   var value = Reader[item];
   PropertyInfo property = properties.Where(x => x.Name == item).FirstOrDefault();
   var type=property.PropertyType;//Or property.ReflectedType
   var cs= value as type // Error: type is variable but is used like a Type
   if (cs!=null){
     ...
    }

Sample 1:

var value = 123;//is int
type = property.GetType();// is double
var x = (double)value;//Can be casted

Sample 2:

var value = "asd";//is string
type = property.GetType();// is double
var x = (double)value;//Can not be casted

if you don't know the type of the variables then you'll need to use reflection or the `dynamic` type. — Ousmane D., Dec 26 '17 at 08:02
You can use `Expression.Convert` as described here: https://stackoverflow.com/a/3647134/5311735 — Evk, Dec 26 '17 at 10:17

Ofir Winegarten · Answer 1 · 2017-12-26T09:22:27.630

2

You can use IsAssignable:

bool isValidCast = type.IsAssignableFrom(value.GetType())

As per the comments about int to double: I've made a mistake in my comment, so i deleted it. int can be implicitly converted to double because there is a predefined implicit conversion, see here

There are many ways to convert or cast from type to type. For example, You can use implicit/explicit conversion, you can use TypeConverter or implement the IConvertible interface.

Now, you have to decide which use case is relevant to you. It can be a bit complex to check them all, especially without knowing the destination type at design time.

edited Dec 26 '17 at 09:22

answered Dec 26 '17 at 08:10

Ofir Winegarten

9,215
2
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in this example type.IsAssignableFrom return false but casting is possible: `int value = 123; double property = 123.200; var type = property.GetType(); bool isValidCast = type.IsAssignableFrom(value.GetType());// false var x = (double)value;//Can be casted` – masoud Dec 26 '17 at 08:40

Sepehr Samiei · Answer 2 · 2017-12-26T08:48:24.393

1

In your code snippet, type is a variable of type Type, hence why it is throwing that error. You can change your code to use Convert.ChangeType() instead. Something like this:

   var value = Reader[item];
   PropertyInfo property = properties.Where(x => x.Name == item).FirstOrDefault();
   var type=property.PropertyType;
   object cs= Convert.ChangeType(value, type);
   if (cs!=null){
     ...
    }

Notice that, since you don't know the strong type of your property at compile time, you still have to box it into an object type after changing its type. This means you wouldn't be able to access its properties and methods directly using dot syntax in code (e.g. cs.MyProperty). If you wish to be able to do that, you can use the dynamic type in C#:

dynamic dcs = cs;
Console.Write(dcs.MyProperty);

When using Convert.ChangeType() you have to make sure you are converting to the correct type. E.g.:

if (value.GetType() == type)
{
   object cs= Convert.ChangeType(value, type);   
}

edited Dec 26 '17 at 08:48

answered Dec 26 '17 at 08:10

Sepehr Samiei

382
1
6

in Convert.ChangeType() If it can not be cast, an error occurs – masoud Dec 26 '17 at 08:26
Yes, if, e.g., you try to convert string to int, it will throw error. But if, e.g., you convert int that is boxed inside object, it will work perfectly. – Sepehr Samiei Dec 26 '17 at 08:43
You have to be more specific about your problem. Have you ascertained the real type of your variable? From your code snippet, it is not clear how the variable value is related to property and its property.PropertyType. You might have acquired value from some source which returns it in string or some other format. In that case you should use other methods to convert it to your desired type. – Sepehr Samiei Dec 26 '17 at 08:55
Suppose the value is int `value=123` and type is `double` . the value has the ability to casting double. but `if (value.GetType() == type)` Makes the cast not happen – masoud Dec 26 '17 at 08:59
Number types are the only exceptions at language level. You can handle that special case in your code. E.g.: if (typeof(value) is int || typeof(value) is double)... – Sepehr Samiei Dec 26 '17 at 09:10

How to use variable type in as operator

2 Answers2