问题描述

使用APIM，在 Inbound 中对请求的Body内容进行解析。客户端请求所传递的Request Body为XML格式，需要从Request Body中解析出多个(Element)节点值，然后设置通过(set-variable)为参数在后续使用。

但是验证发现，当且只当使用一个set-variable 从 Request Body中读取数据时候，是可以成功的。如果要读取第二个，第三个时，始终会遇见一个诡异的错误 Expression evaluation failed. Object reference not set to an instance of an object。关键问题是，为什么第一个可以成功，第二个的语句和第一个完全一样，却面临如此问题？真是诡异！

需要解析的XML格式如下：

<?xml version="1.0" encoding="utf-8"?>
<CDHotel xmlns="http://schemas.xmlsoap.org/soap/cdhotel/">
<Body>
<GetHotel xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://tempuri.org/">
<input>
<ID xmlns="http://schemas.datacontract.org/2014/01/wcf">202203081007001</ID>
<Name xmlns="http://schemas.datacontract.org/2014/01/wcf">Cheng Du Junyi Hotel</Name>
<Code xmlns="http://schemas.datacontract.org/2014/01/wcf">ICP1009100</Code>
</input>
</GetHotel>
</Body>
</CDHotel>

在APIM Policies中，需要获取 ID, Name， Code 和 Desc 值，策略语句如下：

<!-- IMPORTANT:
    - Policy elements can appear only within the <inbound>, <outbound>, <backend> section elements.
    - To apply a policy to the incoming request (before it is forwarded to the backend service), place a corresponding policy element within the <inbound> section element.
    - To apply a policy to the outgoing response (before it is sent back to the caller), place a corresponding policy element within the <outbound> section element.
    - To add a policy, place the cursor at the desired insertion point and select a policy from the sidebar.
    - To remove a policy, delete the corresponding policy statement from the policy document.
    - Position the <base> element within a section element to inherit all policies from the corresponding section element in the enclosing scope.
    - Remove the <base> element to prevent inheriting policies from the corresponding section element in the enclosing scope.
    - Policies are applied in the order of their appearance, from the top down.
    - Comments within policy elements are not supported and may disappear. Place your comments between policy elements or at a higher level scope. -->
<policies>
    <inbound>
        <base />
        <set-variable name="myID" value="@(
 context.Request.Body.As<XElement>().Descendants().FirstOrDefault(x => x.Name.LocalName == "ID")?.Value
        )" />
        <set-variable name="myName" value="@(
 context.Request.Body.As<XElement>().Descendants().FirstOrDefault(x => x.Name.LocalName == "Name")?.Value
        )" />
        <set-variable name="myCode" value="@(
 context.Request.Body.As<XElement>().Descendants().FirstOrDefault(x => x.Name.LocalName == "Code")?.Value
        )" />
        <set-variable name="myDesc" value="@(
 context.Request.Body.As<XElement>().Descendants().FirstOrDefault(x => x.Name.LocalName == "Desc")?.Value
        )" />
    </inbound>
    <backend>
        <base />
    </backend>
    <outbound>
        <set-header name="myID" exists-action="override">
            <value>@((string)context.Variables["myID"])</value>
        </set-header>
        <set-header name="myName" exists-action="override">
            <value>@((string)context.Variables["myName"])</value>
        </set-header>
        <set-header name="myCode" exists-action="override">
            <value>@((string)context.Variables["myCode"])</value>
        </set-header>
        <set-header name="myDesc" exists-action="override">
            <value>@((string)context.Variables["myDesc"])</value>
        </set-header>
        <base />
    </outbound>
    <on-error>
        <base />
    </on-error>
</policies>

在APIM的Test功能，查看Trace语句后，错误消息为：

set-variable (0.905 ms)
    {
    "message": "Expression was successfully evaluated.",
    "expression": "\n context.Request.Body.As<XElement>().Descendants().FirstOrDefault(x => x.Name.LocalName == \"ID\")?.Value\n        ",
    "value": "202203081007001"
}
set-variable (0.013 ms)
    {
    "message": "Context variable was successfully set.",
    "name": "myID",
    "value": "202203081007001"
}
set-variable (7.898 ms)
    {
    "messages": [
        {
            "message": "Expression evaluation failed.",
            "expression": "\n context.Request.Body.As<XElement>().Descendants().FirstOrDefault(x => x.Name.LocalName == \"Name\")?.Value\n        ",
            "details": "Object reference not set to an instance of an object."
        },
        "Expression evaluation failed. Object reference not set to an instance of an object.", "Object reference not set to an instance of an object."
    ]
}

说明：

绿色高亮部分为Set-Variable的语句，两者语法完全一样。
但第二次就出现了未将对象应用到实例的异常。

错误截图：

问题解决

经过反复实验，问题肯定出现在 context.Request.Body.As 上，是不是这个内容只能使用一次呢？经 Google 搜寻，终于得出了官方解释和解决办法：

官方解释

文档链接：https://docs.microsoft.com/en-us/azure/api-management/api-management-policy-expressions#ContextVariables

context.Request.Body.As<T> 和 context.Response.Body.As 方法用As<T>的方式指定读取 Request 和 Response的Body内容，默认情况下，这个方式读取的时原始消息的Body流，读取一次后就变为不可用，也就是说只能 As<T>的方式一次。这就解释了为什么第二个Set Variable语句出现 Object 异常。

解决办法

正如文档中解释，使用 preserveContent : true 后，可以多次转换 Body Stream。

修改后的Policy为：

   <inbound>
        <base />
        <set-variable name="myID" value="@(
 context.Request.Body.As<XElement>(preserveContent:true).Descendants().FirstOrDefault(x => x.Name.LocalName == "ID")?.Value
        )" />
        <set-variable name="myName" value="@(
 context.Request.Body.As<XElement>(preserveContent:true).Descendants().FirstOrDefault(x => x.Name.LocalName == "Name")?.Value
        )" />
        <set-variable name="myCode" value="@(
 context.Request.Body.As<XElement>(preserveContent:true).Descendants().FirstOrDefault(x => x.Name.LocalName == "Code")?.Value
        )" />
        <set-variable name="myDesc" value="@(
 context.Request.Body.As<XElement>(preserveContent:true).Descendants().FirstOrDefault(x => x.Name.LocalName == "Desc")?.Value
        )" />
    </inbound>

修改后，测试解析XML文件动画：

注意：

因为APIM实例的内存存在限制，内部的Memory限制为500MB，当缓存的Request/Response的内容大于500MB的时候，就会出现 MessagePayLoadTooLarge异常。
当使用 preserveContent:true 后，会把当前的Body内容缓存在APIM实例的内存中，如果Body内容大于500MB，则会出现 MessagePayLoadTooLarge问题，所以对于Body Size过大的请求，不能使用 Buffer 及读取整个Response/Request Body在Policy代码中。