'casting' with reflection

Consider the following sample code:

class SampleClass
{
    public long SomeProperty { get; set; }
}

public void SetValue(SampleClass instance, decimal value)
{
    // value is of type decimal, but is in reality a natural number => cast
    instance.SomeProperty = (long)value;
}

Now I need to do something similar through reflection:

void SetValue(PropertyInfo info, object instance, object value)
{
    // throws System.ArgumentException: Decimal can not be converted to Int64
    info.SetValue(instance, value)  
}

Note that I cannot assume that the PropertyInfo always represents a long, neither that value is always a decimal. However, I know that value can be casted to the correct type for that property.

How can I convert the 'value' parameter to the type represented by PropertyInfo instance through reflection ?

void SetValue(PropertyInfo info, object instance, object value)
{
    info.SetValue(instance, Convert.ChangeType(value, info.PropertyType));
}

Thomas answer only works for types that implement IConvertible interface:

> For the conversion to succeed, value must implement the IConvertible interface, because the method simply wraps a call to an appropriate IConvertible method. The method requires that conversion of value to conversionType be supported.

This code compile a linq expression that does the unboxing (if needed) and the conversion:

    public static object Cast(this Type Type, object data)
    {
        var DataParam = Expression.Parameter(typeof(object), "data");
        var Body = Expression.Block(Expression.Convert(Expression.Convert(DataParam, data.GetType()), Type));

        var Run = Expression.Lambda(Body, DataParam).Compile();
        var ret = Run.DynamicInvoke(data);
        return ret;
    }

The resulting lambda expression equals to (TOut)(TIn)Data where TIn is the type of the original data and TOut is the given type

The answer by Thomas is right, but I thought I would add my finding that Convert.ChangeType does not handle conversion to nullable types. To handle nullable types, I used the following code:

void SetValue(PropertyInfo info, object instance, object value)
{
    var targetType = info.PropertyType.IsNullableType() 
         ? Nullable.GetUnderlyingType(info.PropertyType) 
         : info.PropertyType; 
    var convertedValue = Convert.ChangeType(value, targetType);

    info.SetValue(instance, convertedValue, null);
}

This code makes use of the following extension method:

public static class TypeExtensions
{
    public static bool IsNullableType(this Type type)
    {
        return type.IsGenericType
               && type.GetGenericTypeDefinition().Equals(typeof(Nullable<>));
    }
}

Contributing to jeroenh's answer, I would add that Convert.ChangeType crashes with a null value, so the line for getting the converted value should be:

var convertedValue = value == null ? null : Convert.ChangeType(value, targetType);

When the Type is a Nullable Guid then none of the above proposed solutions work. Invalid cast from 'System.DBNull' to 'System.Guid' exception is thrown at Convert.ChangeType

To fix that change to:

var convertedValue = value == System.DBNull.Value ? null : Convert.ChangeType(value, targetType);

This is a very old question but I thought I'd chime in for ASP.NET Core Googlers.

In ASP.NET Core, .IsNullableType() is protected (amongst other changes) so the code is a tad different. Here's @jeroenh's answer modified to work in ASP.NET Core:

void SetValue(PropertyInfo info, object instance, object value)
{
    Type proptype = info.PropertyType;
    if (proptype.IsGenericType && proptype.GetGenericTypeDefinition().Equals(typeof(Nullable<>)))
    {
        proptype = new NullableConverter(info.PropertyType).UnderlyingType;
    }

    var convertedValue = Convert.ChangeType(value, proptype);
    info.SetValue(instance, convertedValue);
}