static Expression ReadValue(ParameterExpression input, Mapping <Thing, Thing> map)
{
Expression value = Expression.PropertyOrField(input, map.From.Name);
var fromType = map.From.Type;
var toType = map.To.Type;
if (fromType == toType)
{
return(value);
}
if (Types.CanBeCast(fromType, toType)) // e.g. int to long
{
return(Expression.Convert(value, toType));
}
if (Types.IsNullable(fromType))
{
Expression fromValueOrDefault = Expression.Call(value, fromType.GetMethod("GetValueOrDefault", Type.EmptyTypes));
var fromArgType = Types.NullableOf(fromType);
if (fromArgType == toType) // e.g. int? to int
{
return(fromValueOrDefault);
}
var cast = Types.GetExplicitCastOperator(fromArgType, toType);
if (cast != null) // e.g. IntId<>? to int
{
return(Expression.Call(cast, fromValueOrDefault));
}
if (Types.IsNullable(toType))
{
// nullable<> to nullable<> conversion must handle null to null as a special case
var toArgType = toType.GetGenericArguments()[0];
if (Types.CanBeCast(fromArgType, toType)) // e.g. int? to long?
{
return(Expression.Condition(Expression.PropertyOrField(value, "HasValue"), Expression.Convert(fromValueOrDefault, toType), Expression.Default(toType)));
}
cast = Types.GetExplicitCastOperator(fromArgType, toArgType); // e.g. IntId<>? to int?
if (cast != null)
{
fromValueOrDefault = Expression.Call(cast, fromValueOrDefault);
}
return(Expression.Condition(Expression.PropertyOrField(value, "HasValue"), Expression.Convert(fromValueOrDefault, toType), Expression.Default(toType)));
}
if (Types.CanBeCast(fromArgType, toType)) // e.g. int? to long
{
return(Expression.Convert(fromValueOrDefault, toType));
}
}
if (Types.IsNullable(toType))
{
var toArgType = toType.GetGenericArguments()[0];
var cast2 = Types.GetExplicitCastOperator(fromType, toArgType); // e.g. IntId<> to int?
if (cast2 != null)
{
return(Expression.Convert(Expression.Call(cast2, value), toType));
}
}
var cast3 = Types.GetExplicitCastOperator(fromType, toType); // e.g. IntId<> to int
if (cast3 != null)
{
return(Expression.Call(cast3, value));
}
throw new InvalidOperationException("Should never get here has types compatibility has been checked");
}