数据类型转换函数

Background

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.

 

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
[MethodImpl(MethodImplOptions.Synchronized)]
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(
Expression.Convert(
Expression.Convert(
Expression.Convert(
convParameter
, fromType
)
, targetType
)
, typeof(object)
)
, convParameter
).Compile();
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);