无法将“NSNull”类型的值转换为“NSDictionary”答案

【问题标题】：Could not cast value of type 'NSNull' to 'NSDictionary'无法将“NSNull”类型的值转换为“NSDictionary”
【发布时间】：2018-02-08 22:10:15
【问题描述】：

我在“self.email = (snapshot.value as!NSDictionary)["email"] as!String”行中收到一个错误，说它无法转换类型为“NSNull”的值。如何修复错误？我认为既然我有一个 toAnyObject 函数，它会将所有内容都转换为字符串。

struct User {
    var email: String!
    var uid: String!
    var ref: DatabaseReference!
    var key: String = ""

    init(snapshot: DataSnapshot) {
        self.email = (snapshot.value as! NSDictionary)["email"] as! String
        self.uid = (snapshot.value as! NSDictionary)["uid"] as! String
        self.ref = snapshot.ref
        self.key = snapshot.key
    }

    init(email: String, uid: String) {
        self.email = email
        self.uid = uid
        self.ref = Database.database().reference()
    }

    func toAnyObject() -> [String: Any] {
        return ["email":self.email, "uid":self.uid]
    }
}

【问题讨论】：

if let snap = snapshot.value as? [String:Any] { self.email = snap["email"] as! String } 不要强制解包，因为它可能是空的，就像你的情况一样 ..snapshot.value 是空的
见此例如stackoverflow.com/questions/36253870/…
如果我想添加另一个字段，我应该简单地添加吗？出于某种原因，该代码给了我一个错误。如果让 snap = snapshot.value as？ [String:Any] { self.email = snap["email"] as！字符串 self.firstname = snap["firstname"] as!字符串 }

标签： ios swift xcode dictionary swift3

【解决方案1】：

正如 El Captain v2.0 指出的那样，不要强行打开一个值，因为它可能是 nil，你会遇到崩溃。请改用if let：

// if snap has a value, then you can use the value. Otherwise not, you can call an else if you want to.
if let snap = snapshot.value as? [String:Any] { 
    self.email = snap["email"] as! String 
}

我也会跳过as! String，而是这样做，以防你没有得到email的值。

if let snap = snapshot.value as? [String:Any], let email = snap["email"] as? String  { 
    // Use email in here now
    print(email)
}

因此，如果您有其他想要获取的字段，只需将它们添加到if-statement。

【讨论】：

【解决方案2】：

    struct User {

    var email: String!
    var uid: String!
    var ref: DatabaseReference!
    var key: String = ""

    init(snapshot: DataSnapshot){

        self.email = (snapshot.value as! NSDictionary)["email"] as! String
        self.uid = (snapshot.value as! NSDictionary)["uid"] as! String
        self.ref = snapshot.ref
        self.key = snapshot.key

    }

    init(email: String, uid: String){

        if let snap = snapshot.value as? [String:Any] {
        // Do not unwrap forcefully, instead use as mentioned below 
        // so that in case snap["email"] gives null that could be 
        //handled. 
        //Also please check that whether you have "firstname" var 
        //present in not in your code and dictionary both. 
        self.email = snap["email"] as? String ?? "" //Provide some default value or empty string
        }
        self.uid = uid
        self.ref = Database.database().reference()

    }

    func toAnyObject() -> [String: Any]{
        return ["email":self.email, "uid":self.uid]
    }
    }

【讨论】：