如何将两个C# Agreements类型变量合并为一个实例？

你遇到的问题大概率是原代码未处理列表为null的情况，或是合并需求涉及去重但原代码未实现。以下是几种可行的解决方式：

1. 处理空列表的基础合并 #

如果只是单纯合并两个实例的列表，同时避免null引发的异常，可使用空合并运算符(??)确保列表不为空：

Agreements combinedSTDStatePlansAgreements = new Agreements
{
    clientEnterpriseId = agreementsSTD.clientEnterpriseId,
    agreements = (agreementsSTD.agreements ?? new List<Agreement>())
        .Concat(agreementsStatePlans.agreements ?? new List<Agreement>())
        .ToList(),
    serviceAgreements = (agreementsSTD.serviceAgreements ?? new List<ServiceAgreement>())
        .Concat(agreementsStatePlans.serviceAgreements ?? new List<ServiceAgreement>())
        .ToList()
};

这种方式会保留所有元素（包括重复项），同时避免因某个列表为null导致的空引用异常。

2. 合并并去重（基于唯一标识） #

如果需要合并后去除重复的Agreement或ServiceAgreement对象，可自定义比较器结合Union方法实现：
首先定义比较器（假设对象有唯一标识属性Id）：

// Agreement比较器
public class AgreementEqualityComparer : IEqualityComparer<Agreement>
{
    public bool Equals(Agreement x, Agreement y)
    {
        return x?.Id == y?.Id;
    }

    public int GetHashCode(Agreement obj)
    {
        return obj?.Id.GetHashCode() ?? 0;
    }
}

// ServiceAgreement比较器
public class ServiceAgreementEqualityComparer : IEqualityComparer<ServiceAgreement>
{
    public bool Equals(ServiceAgreement x, ServiceAgreement y)
    {
        return x?.Id == y?.Id;
    }

    public int GetHashCode(ServiceAgreement obj)
    {
        return obj?.Id.GetHashCode() ?? 0;
    }
}

然后执行合并去重：

Agreements combinedSTDStatePlansAgreements = new Agreements
{
    clientEnterpriseId = agreementsSTD.clientEnterpriseId,
    agreements = (agreementsSTD.agreements ?? new List<Agreement>())
        .Union(agreementsStatePlans.agreements ?? new List<Agreement>(), new AgreementEqualityComparer())
        .ToList(),
    serviceAgreements = (agreementsSTD.serviceAgreements ?? new List<ServiceAgreement>())
        .Union(agreementsStatePlans.serviceAgreements ?? new List<ServiceAgreement>(), new ServiceAgreementEqualityComparer())
        .ToList()
};

3. 手动添加元素（更直观的方式） #

如果觉得LINQ的写法不够直观，也可以手动初始化列表并添加元素：

Agreements combinedSTDStatePlansAgreements = new Agreements
{
    clientEnterpriseId = agreementsSTD.clientEnterpriseId,
    agreements = new List<Agreement>(),
    serviceAgreements = new List<ServiceAgreement>()
};

// 添加agreements列表元素
if (agreementsSTD.agreements != null)
{
    combinedSTDStatePlansAgreements.agreements.AddRange(agreementsSTD.agreements);
}
if (agreementsStatePlans.agreements != null)
{
    combinedSTDStatePlansAgreements.agreements.AddRange(agreementsStatePlans.agreements);
}

// 添加serviceAgreements列表元素
if (agreementsSTD.serviceAgreements != null)
{
    combinedSTDStatePlansAgreements.serviceAgreements.AddRange(agreementsSTD.serviceAgreements);
}
if (agreementsStatePlans.serviceAgreements != null)
{
    combinedSTDStatePlansAgreements.serviceAgreements.AddRange(agreementsStatePlans.serviceAgreements);
}

原代码失效的常见原因 #

• 其中一个实例的agreements或serviceAgreements为null，调用Concat时抛出空引用异常；
• 需要去重但原代码未处理，导致合并结果包含重复对象，不符合预期。

内容的提问来源于stack exchange，提问作者Letoncse