转换Expression.Property结果

Question

如何Expression动态创建树时创建强制转换？

问题是,我有一个类型的属性string:

public class Test
{
    public string Id { get; set; }
}

我想一般创建一个强类型的lambda表达式,表示一个委托,它返回一个object而不是a string(Expression<Func<T, object>>).

现在我这样做:

private static Expression<Func<T, object>> CreateIdQuery()
{
    Type type = typeof(T);
    PropertyInfo idProperty = type.GetProperty("Id");
    ParameterExpression lambdaParam = Expression.Parameter(type, "x");
    MemberExpression body = Expression.Property(lambdaParam, idProperty);
    LambdaExpression expr = Expression.Lambda(body, lambdaParam);
    return (Expression<Func<T, object>>)expr;
}

但它在最后一行引发异常(我无法Expression<Func<Test, string>>转换Expression<Func<Test, object>>).

我如何施放表达式的主体(我假设该MemberExpression部分需要被投入一个object)？

Answer 1

使用Expression.Convert(body, typeof(object)).

private static Expression<Func<T, object>> CreateIdQuery()
{
    Type type = typeof(T);
    PropertyInfo idProperty = type.GetProperty("Id");
    ParameterExpression lambdaParam = Expression.Parameter(type, "x");
    MemberExpression body = Expression.Property(lambdaParam, idProperty);
    UnaryExpression converted = Expression.Convert(body, typeof(object));
    LambdaExpression expr = Expression.Lambda(converted, lambdaParam);
    return (Expression<Func<T, object>>)expr;
}