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Using LINQ for type conversion

, 25 Feb 2010 CPOL 25.7K 30
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Converting between types in .NET.


.NET provides several ways to change the type of a value to another type at run-time. Each technique has its limitations. This article provides (yet) another alternative that works in all instances when the CLR is capable of performing a type cast.


Two popular solutions to the problem of type conversion are to use System.Convert.ChangeType, or to obtain System.ComponentModel.TypeConverter and call its ConvertFrom method. The first method breaks when you try converting a value type T to System.Nullable<T>; the second one breaks when you try converting different numeric types, for example, float to double. These limitations appear especially frustrating, because the CLR has built-in capabilities to perform both types of conversion.

One way of using these type casting capabilities is to build a LINQ lambda expression, compile it into a Func<object,object>, and then use the compiled delegate every time we need to convert between two types.

Using the code

The code below implements this approach, wrapping it into an extension method of System.Type.

public static class TypeCast {
    // This is the method exposing the rest of the functionality
    public static object Cast(this Type type, object obj) {
        return GetConverter(type, obj)(obj);
    private static readonly IDictionary<PairOfTypes,Func<object,object>> converters =
        new Dictionary<PairOfTypes,Func<object,object>>();
    private static readonly ParameterExpression convParameter =
        Expression.Parameter(typeof(object), "val");
    // This is the method with the "guts" of the implementation
    private static Func<object,object> GetConverter(Type targetType, object val) {
        var fromType = val != null ? val.GetType() : typeof(object);
        var key = new PairOfTypes(fromType, targetType);
        Func<object,object> res;
        if (converters.TryGetValue(key, out res)) {
            return res;
        res = (Func<object,object>)Expression.Lambda(
                    ,   fromType
                ,   targetType
            ,   typeof(object)
        ,   convParameter
        converters.Add(key, res);
        return res;
    // This class provides Equals and GetHashCode
    // for a pair of System.Type objects.
    private class PairOfTypes {
        private readonly Type first;
        private readonly Type second;
        public PairOfTypes(Type first, Type second) {
            this.first = first;
            this.second = second;
        public override int GetHashCode() {
            return 31*first.GetHashCode() + second.GetHashCode();
        public override bool Equals(object obj) {
            if (obj == this) {
                return true;
            var other = obj as PairOfTypes;
            if (other == null) {
                return false;
            return first.Equals(other.first)
                && second.Equals(other.second);

Now, you can use the Cast method like this:

double? x = typeof(double?).Cast(1.0);
int y = typeof(int).Cast(1.2345);

Happy coding!


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


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Comments and Discussions

GeneralMy vote of 5 Pin
Jörgen Andersson14-Nov-12 2:24
memberJörgen Andersson14-Nov-12 2:24 
GeneralI cant get a simple example to build Pin
theperm9-Feb-11 5:55
membertheperm9-Feb-11 5:55 
GeneralRe: I cant get a simple example to build Pin
dasblinkenlight9-Feb-11 6:28
memberdasblinkenlight9-Feb-11 6:28 
Generalgood one - have 5 from me Pin
Pranay Rana19-Dec-10 0:30
memberPranay Rana19-Dec-10 0:30 
GeneralInteresting thing... Pin
Paulo Zemek26-Feb-10 6:17
memberPaulo Zemek26-Feb-10 6:17 
GeneralRe: Interesting thing... Pin
dasblinkenlight26-Feb-10 11:23
memberdasblinkenlight26-Feb-10 11:23 
QuestionWhat am I missing? Pin
Josh Fischer25-Feb-10 8:59
mvpJosh Fischer25-Feb-10 8:59 
AnswerRe: What am I missing? Pin
dasblinkenlight25-Feb-10 9:31
memberdasblinkenlight25-Feb-10 9:31 
This is a solution for performing a CLR cast in situations when neither the target type nor the type of the value being converted is known statically (i.e. at compile time).

Consider a situation when you read a row from a database, and populate properties of a .NET object with its data. The query comes as a string passed to your program at run-time, and the object comes as an instance of System.Object.

The algorithm would go like this:

for each field returned from the query
1. Use reflection to get a property setter corresponding to the name of the query field
2. Get the value of the field from the SqlDataReader
3. Convert the data to the type of the corresponding property
4. Call the property setter through reflection, passing it the converted value

Step #3 is necessary if you want your solution to be general. Without this step, reading a SQL float into a .NET float would result in an exception, because SqlDataReader converts SQL floats to double, among other things.
GeneralRe: What am I missing? Pin
Josh Fischer25-Feb-10 14:05
mvpJosh Fischer25-Feb-10 14:05 
AnswerRe: What am I missing? Pin
dasblinkenlight25-Feb-10 18:30
memberdasblinkenlight25-Feb-10 18:30 
GeneralRe: What am I missing? Pin
Josh Fischer26-Feb-10 4:18
mvpJosh Fischer26-Feb-10 4:18 
GeneralGetHashCode and the secrets of multiplying by 31 Pin
Marc Brooks1-Mar-10 13:51
memberMarc Brooks1-Mar-10 13:51 
GeneralRe: What am I missing? Pin
PIEBALDconsult26-Feb-10 3:23
mvpPIEBALDconsult26-Feb-10 3:23 
AnswerRe: What am I missing? Pin
dasblinkenlight26-Feb-10 5:47
memberdasblinkenlight26-Feb-10 5:47 
GeneralInteresting, but... Pin
Paulo Zemek25-Feb-10 8:57
memberPaulo Zemek25-Feb-10 8:57 
GeneralRe: Interesting, but... Pin
dasblinkenlight25-Feb-10 9:45
memberdasblinkenlight25-Feb-10 9:45 
GeneralRe: Interesting, but... [modified] Pin
Paulo Zemek25-Feb-10 10:34
memberPaulo Zemek25-Feb-10 10:34 
GeneralRe: Interesting, but... Pin
dasblinkenlight25-Feb-10 18:01
memberdasblinkenlight25-Feb-10 18:01 

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