Dynamics 365 C#插件用FetchXML触发Object reference空引用错误

我正在用C#开发Dynamics 365插件，尝试通过FetchXML发起数据查询请求时，遇到错误：Object reference not set to an instance of an object.。以下是我的代码：

public void Execute(IServiceProvider serviceProvider)
{
    // Obtain the tracing service
    ITracingService tracingService =
        (ITracingService)serviceProvider.GetService(typeof(ITracingService));

    // Obtain the execution context from the service provider.  
    IPluginExecutionContext context = (IPluginExecutionContext)
        serviceProvider.GetService(typeof(IPluginExecutionContext));

    IOrganizationService svc = (IOrganizationService)serviceProvider.GetService(typeof(IOrganizationService));

    try
    {
        var id = context.InputParameters["leadid"] as string;

        var fetchXml = $@"<?xml version=""1.0"" encoding=""utf-16""?>
            <fetch>
              <entity name=""lead"">
                <attribute name=""leadid"" />
                <link-entity name=""new_sitecorevisit"" from=""new_new_parent_leadid"" to=""leadid"">
                  <attribute name=""new_visitduration"" />
                  <filter>
                    <condition attribute=""new_new_parent_leadid"" operator=""eq"" value=""{id}"" />
                  </filter>
                </link-entity>
              </entity>
            </fetch>";

        EntityCollection result = svc.RetrieveMultiple(new FetchExpression(fetchXml));

        context.OutputParameters["data"] = result;
    }

    catch (Exception ex)
    {
        tracingService.Trace("Error: {0}", ex.ToString());
        throw;
    }
}

我认为错误出现在EntityCollection result = svc.RetrieveMultiple(new FetchExpression(fetchXml));这一行，移除该行后问题消失。我已检查过fetchXml变量和id变量，均无异常；且通过JavaScript调用可正常获取数据，但这对.NET插件开发无用。请问我哪里出错了？

1. 修正IOrganizationService实例获取方式 #

直接通过serviceProvider.GetService(typeof(IOrganizationService))获取服务会返回null，正确流程是先获取服务工厂再创建实例：

// 替换原有的svc获取代码
IOrganizationServiceFactory serviceFactory = (IOrganizationServiceFactory)serviceProvider.GetService(typeof(IOrganizationServiceFactory));
IOrganizationService svc = serviceFactory.CreateOrganizationService(context.UserId);

2. 修复FetchXML格式问题 #

C#字符串模板中无需转义XML标签，直接使用<>即可，同时建议将编码改为utf-8避免兼容性问题：

var fetchXml = $@"<?xml version=""1.0"" encoding=""utf-8""?>
    <fetch>
      <entity name=""lead"">
        <attribute name=""leadid"" />
        <link-entity name=""new_sitecorevisit"" from=""new_new_parent_leadid"" to=""leadid"">
          <attribute name=""new_visitduration"" />
          <filter>
            <condition attribute=""new_new_parent_leadid"" operator=""eq"" value=""{leadId.ToString()}"" />
          </filter>
        </link-entity>
      </entity>
    </fetch>";

3. 修正LeadId类型转换 #

leadid在Dynamics 365中是Guid类型，不能直接用as string转换，需做安全校验：

// 替换原有的id获取代码
Guid leadId;
if (!context.InputParameters.TryGetValue("leadid", out var leadIdObj) || !(leadIdObj is Guid))
{
    tracingService.Trace("leadid参数缺失或类型错误");
    throw new InvalidPluginExecutionException("无效的leadid参数");
}
leadId = (Guid)leadIdObj;

4. 调整输出参数赋值 #

插件输出参数不支持直接传入EntityCollection，改为传入实体列表：

context.OutputParameters["data"] = result.Entities;

内容的提问来源于stack exchange，提问作者dwewers