c# - 构建表达式以使用 DateTime 调用 StringBuilder.Append(Object) 时出现异常

Question

我构建了一个 ToStringBuilder(发现 here)，它反射(reflect)类型并动态构建 Expression 以编译快速 ToString 方法。

它运行良好，但我刚刚发现它在 DateTimes 上出错。当尝试构建对传递 DateTime 的 StringBuilder.Append(Object) 的调用时，它会阻塞。我是否需要为框值类型创建表达式？如何做到最好？

我创建了以下测试用例来演示失败。

    // passes
    [Test]
    public void AppendDateTime()
    {
        StringBuilder sb = new StringBuilder();
        sb.Append(new DateTime());
    }


    // throws 
    [Test]
    public void ExpressionAppendDateTime()
    {
        ParameterExpression sbArgExpression = Expression.Parameter(typeof(StringBuilder), "sb");
        ParameterExpression dateTimeArgExpression = Expression.Parameter(typeof(DateTime), "dateTime");

        var appendMethod = typeof(StringBuilder).GetMethod("Append", new[] {typeof(DateTime)});
        var call = Expression.Call(sbArgExpression, appendMethod, dateTimeArgExpression);

        // throws on this line
        var lambda = Expression.Lambda<Action<StringBuilder, DateTime>>(call, sbArgExpression, dateTimeArgExpression).Compile();  

        var datetime = new DateTime();
        var sb = new StringBuilder();
        lambda.Invoke(sb, datetime);
    }

异常(exception)是..

System.ArgumentException was unhandled by user code
  Message=Expression of type 'System.DateTime' cannot be used for parameter of type 'System.Object' of method 'System.Text.StringBuilder Append(System.Object)'
  Source=System.Core
  StackTrace:
       at System.Linq.Expressions.Expression.ValidateOneArgument(MethodBase method, ExpressionType nodeKind, Expression arg, ParameterInfo pi)
       at System.Linq.Expressions.Expression.ValidateArgumentTypes(MethodBase method, ExpressionType nodeKind, ReadOnlyCollection`1& arguments)
       at System.Linq.Expressions.Expression.Call(Expression instance, MethodInfo method, IEnumerable`1 arguments)
       at System.Linq.Expressions.Expression.Call(Expression instance, MethodInfo method, Expression[] arguments)
       at Tests.TestToStringBuilder.ExpressionAppendDateTime() in 
  InnerException: 

Answer 1

已解决，必须使用 Expression.TypeAs 将非原始值类型键入为 Object

    [Test]
    public void ExpressionAppendDateTime()
    {
        ParameterExpression sbArgExpression = Expression.Parameter(typeof(StringBuilder), "sb");
        ParameterExpression dateTimeArgExpression = Expression.Parameter(typeof(DateTime), "dateTime");

        var appendMethod = typeof(StringBuilder).GetMethod("Append", new[] {typeof(DateTime)});

        Type t = typeof(DateTime);
        Expression arg;
        if (t.IsValueType && !t.IsPrimitive)
        {
            arg = Expression.TypeAs(dateTimeArgExpression, typeof(object));
        }
        else
        {
            arg = dateTimeArgExpression;
        }

        var call = Expression.Call(sbArgExpression, appendMethod, arg);

        var lambda = Expression.Lambda<Action<StringBuilder, DateTime>>(call, sbArgExpression, dateTimeArgExpression).Compile();

        var datetime = new DateTime();
        var sb = new StringBuilder();
        lambda.Invoke(sb, datetime);
    }