如何解决 The given key was not present in the dictionary 错误？答案

【问题标题】：How to solve The given key was not present in the dictionary error?如何解决 The given key was not present in the dictionary 错误？
【发布时间】：2017-02-22 11:21:40
【问题描述】：

所以。我已经花了 3 个小时试图找出引发 The given key was not present in the dictionary. 错误的问题，但不幸的是，通过许多 SO 问题和博客文章并没有太大帮助。

我有一个简单的 Facebook 批处理参数字典，当我遍历结果时，它会抛出错误 The given key was not present in the dictionary。

这是引发异常的代码：

foreach (FacebookPermissionModel permissionResult in
    result.SelectMany(x => x.data).Where(y => y.status == "granted"))
{
    RequiredPermissions[permissionResult.permission].granted = true; // the error comes from here.
}

完整代码如下：

protected List<FacebookPermissionRequest> CheckPermissions(Dictionary<string, FacebookPermissionRequest> RequiredPermissions)
{
    var access_token = HttpContext.Items["access_token"].ToString();
    if (!string.IsNullOrEmpty(access_token))
    {
        var appsecret_proof = access_token.GenerateAppSecretProof();
        var fb = new FacebookClient(access_token);

        IEnumerable<FacebookPermissionRequest> MissingPermissions =
            new List<FacebookPermissionRequest>();  //initialize to an empty list
        if (MissingPermissions != null)
        {
            //create an array of Facebook Batch Parameters based on list of RequiredPermission
            FacebookBatchParameter[] fbBatchParameters =
                new FacebookBatchParameter[RequiredPermissions.Values.Count()];
            int idx = 0;
            foreach (FacebookPermissionRequest required_permssion in
                RequiredPermissions.Values)
            {

                fbBatchParameters[idx] = new FacebookBatchParameter
                {
                    HttpMethod = HttpMethod.Get,
                    Path = string.Format("{0}{1}",
                                            "me/permissions/",
                                            required_permssion.permision_scope_value)
                            .GraphAPICall(appsecret_proof)
                };
                required_permssion.granted = false; //initalize all granted indicators to false for each required permission
                idx++;
            }
            dynamic permission_Batchresult = fb.Batch(
                fbBatchParameters
            );

            if (permission_Batchresult != null)
            {
                List<PermissionResults> result = JsonConvert.
                    DeserializeObject<List<PermissionResults>>
                    (permission_Batchresult.ToString());

                foreach (FacebookPermissionModel permissionResult in
                    result.SelectMany(x => x.data).Where(y => y.status == "granted"))
                {
                    RequiredPermissions[permissionResult.permission].granted = true; // the error comes from here.
                }
                MissingPermissions = RequiredPermissions.Values.
                    Where(p => p.granted == false);
            }
        }
        return MissingPermissions.ToList();
    }
    else
        throw new HttpException(404, "Missing Access Token");
}

【问题讨论】：

@TimSchmelter 查看我指出的代码引发错误的位置。是的，我确实对其进行了调试，并显示了两个值
既然您已经阅读了有关 stackoverflow 的其他问题，我想您已经知道如何使用 ContainsKey 等来避免这种异常。但问题是：您为什么希望每个 permission 都以输入字典？
只是因为找到丢失的权限，没有授予的权限

标签： c# key

【解决方案1】：

在尝试修改字典之前，您应该检查字典是否包含所需的值。

foreach (FacebookPermissionModel permissionResult in
                        result.SelectMany(x => x.data).Where(y => y.status == "granted"))
{
    if (RequiredPermissions.ContainsKey(permissionResult.permission))
    {
        RequiredPermissions[permissionResult.permission].granted = true;
    {
}

【讨论】：

【解决方案2】：

some permissions not there in RequiredPermissions

            if (RequiredPermissions.ContainsKey(permissionResult.permission))
                RequiredPermissions[permissionResult.permission].granted = true; // the error comes from here.
            else
                Debug.WriteLint("RequiredPermissions obj does not contain this key " + permissionResult.permission);

【讨论】：