Flutter: There should be exactly one item with [DropdownButton]'s value

Well, since no problem has an exact same solution. I was facing the same issue with my code. Here is How I fixed this.

CODE of my DropdownButton:

DropdownButton(
   items: _salutations
         .map((String item) =>
             DropdownMenuItem<String>(child: Text(item), value: item))
         .toList(),
    onChanged: (String value) {
       setState(() {
         print("previous ${this._salutation}");
         print("selected $value");
         this._salutation = value;
            });
          },
     value: _salutation,
),

The Error

In the code snippet below, I am setting the state for a selection value, which is of type String. Now problem with my code was the default initialization of this selection value. Initially, I was initializing the variable _salutation as:

String _salutation = ""; //Notice the empty String.

This was a mistake!

Initial selection should not be null or empty as the error message correctly mentioned.

'items == null || items.isEmpty || value == null ||

And hence the crash:

Solution
Initialize the value object with some default value. Please note that the value should be the one of the values contained by your collection. If it is not, then expect a crash.

  String _salutation = "Mr."; //This is the selection value. It is also present in my array.
  final _salutations = ["Mr.", "Mrs.", "Master", "Mistress"];//This is the array for dropdown

Might also get this error if trying to set value of dropdown with a class instance;

  var tag1 = Tag();
  var tag2 = Tag();
  print(tag1 == tag2); // prints false, dropwdown computes that value is not present among dropdown options

To solve this override operator ==:

class Tag{
 String name = "tag";

  @override
  bool operator ==(Object other) => other is Tag && other.name == name;

  @override
  int get hashCode => name.hashCode;
}

or use https://pub.dev/packages/equatable lib

class Tag extends Equatable{
 String name = "tag";

  @override
  List<Object> get props => [name];
}

I had the same problem. The solution is simple: you have to be sure that the String that is your default dropdownvalue is contained in the list that you want to use in your dropdownmenu. If you wanted to, let’s say, use a list from an api, you should be sure to know at least one value of that list, so that you could assign it to the variable that is your default dropdownvalue.

Here I want display a list that I obtain from an api. In order to not obtain the error, I set my defaultdropdownvalue with the name ‘Encajes’ that is one of the existing categories that my list contains.

String dropdownValue = "Encajes";

    items: categoriesString
    .map<DropdownMenuItem<String>>((String value) {
  return DropdownMenuItem<String>(
    value: value,
    child: Text(value),
  );
}).toList(),

Code of my dropdown

child: DropdownButton(
      items: _currencies.map((String value) {
        return DropdownMenuItem<String>(
          child: Text(value),
          value: value,
        );
      }).toList(),
      value: 'Rupees',
      onChanged: (String newValueSelected) {
        // Your code to execute, when a menu item is selected from 
        dropdown
      },
))
var _currencies = ['Rupee','Dollar','Pound'];

I faced same error because the value in the dropdown code block is not matching with any of the fields in _currencies

just make the tag class extend from Equatable and pass the attributes to the props.. this did the trick for me.

class Tag extends Equatable{
  String id;
  String name;

  Tag(this.id, this.name);

  @override
  List<Object> get props => [id,name];

}

Okay, some of the answers on this thread will definitely help you resolve the issue. But it is important to clarify why this issue occurs in the first place and what the DropdownButton expects from you. To give you a little background on the issue it is important to understand how two instances of dart objects are compared.

You will very likely not see the above error if your DropdownButton is dealing with a List of int, String, bool, etc.

This is because you can directly compare primitive types and you would get the expected result.

for instance

int x = 5;
int z = 10;
int y = 5;
String foo= 'hello';
String bar = 'hello; 

x == z; // false
x == y; // true
foo == bar; // true

But when dealing with Custom Objects you have to be extra careful and you must ensure you override the "==" operator so that dart knows how to compare instances of your custom object. By default, two objects are equal if they are of the same instance.

consider the Tag class,

class Tag{
final String name;
final String code;

Tag({this.name,this.code});
}

final tag1 = Tag(name:'foo', code: 'hello');
final tag2 = Tag(name:'foo', code: 'hello');
Tag tag3 = tag1;

when you compare tag3==tag1 dart would return true as expected, But when you compare tag1 == tag2, the dart would return false, since both objects are not of the same instance.

So to deal with this issue you need to override the == operator as shown below

class Tag{
final String name;
final String code;

Tag({this.name,this.code});

@override
  bool operator ==(Object other){
      return identical(this, other) ||
        (other.runtimeType == runtimeType &&
        other is Tag &&
        other.name == name &&
        other.code == code
   }
}

Now when you compare tag1 ==tag2 it would return true.

This is documented in the official docs here https://dart.dev/guides/language/effective-dart/design#equality

Coming to the DropdownButton error it expects

1. items is not null
2. items is not empty
3. value is not null
4. value must be present only once in items

Point 4 would fail if you are using Custom objects without overriding the == operator and hence you would get the above error.

TLDR;

So to deal with the error, ensure the above 4 points satisfy and override the == operator so that dart can compare instances of your Tag class as you would expect.

I have had the same issue and surprisingly, there were duplicates in my list of items which were being fetched from a remote DB.

Each time I fetched the data from the server (when a new app user logged in), the data had no duplicates but the same data was being added to the list multiple times because I was logging in multiple users on the same device. Maybe your bug is something similar.

So, make sure you remove any duplicates in the snapshot.data before setting them as items of the DropDownButton.

So I found a solution.

I created empty List to hold my Tag objects.

List<Tag> _tagList = [];

Then, in my initState i assigned the list i get from database to the previous List

     @override
    void initState() {
    super.initState();
    _tagDatabaseHelper.getTagList().then((foo) {
      setState(() {
        _tagList = foo;
      });
    });
  }

Finally My DropdownButton code :

DropdownButton<Tag>(
            isExpanded: true,
            icon: Icon(
              Icons.arrow_drop_down,
              size: 24,
            ),
            hint: Text(
              "Select tags",
              style: TextStyle(color: Color(0xFF9F9F9F)),
            ),
            items: _tagList.map((foo) {
              return DropdownMenuItem(
                value: foo,
                child: Text(foo.tagTitle),
              );
            }).toList(),
            onChanged: (value) {
              setState(() {
                _selectedTag = value;
              });
            },
            value: _selectedTag,
          ),

i had the same Error and my default value was not present in the listItems was mapping in the Dropdown Button as :

String defaultvalue = 'selectCategorie'

const List<String> Subcategories = ['category 1','category 2','category 3'...];

Had to Change to this :-

String defaultvalue = 'selectCategorie';

const List<String> Subcategories = ['selectCategorie','category 1','category 2','category 3'...];

now when you pass the defaultvalue in the DropdownButton no errors

DropdownButton (
  item:[]
 onChanged: (String values){
   print(values);
setState(() {
defaultValue = values;
});
},
value: defaultValue,
)

you can avoid the null value using a ternary operator:

  Container(
             child:
              new DropdownButton<String>(
              value: dropdownValue ?? "1",
              icon: const Icon(Icons.arrow_downward),
              iconSize: 24,
              elevation: 16,
              style: const TextStyle(color: Colors.black, fontSize: 18),
              underline: Container(height: 2, color: Colors.white24, ),
              items: <String>['1', '2', '3', '5'].map((String value) {
              return new DropdownMenuItem<String>(
              value: value,
              child: new Text(value),
              );}).toList(),
              onChanged: (value) {
                  setState(() { dropdownValue=value;});
              },
          )),

I used a trick. The selected item make as first index item in the list .So when changing item at every time remove the item from list and reinsert the item as first item in the list . Please refer the below code. Here iam using Object as the drop down item and the widget i make it as extracted function. and also before calling the dropDownButton function make

//items list like below

 List<LeaveType> items = [
 (id=1,name="Sick"),
 (id=2,name="Paid")
 ]

selectedLeave = null;

Row leaveTypeDropDown(StateSetter setCustomState, List<LeaveType> items) {
    if(selectedLeave != null){
      items.remove(selectedLeave);
      items.insert(0, selectedLeave);
    }
    return Row(
            mainAxisAlignment: MainAxisAlignment.spaceEvenly,
            children : [
              text("Select Leave Type",textSize: 15),
              Padding(padding: const EdgeInsets.all(5)),
              Expanded(
                child: Container(
                  padding: const EdgeInsets.only(left: 10.0, right: 10.0),
                  decoration: BoxDecoration(
                    border: Border.all(color: Colors.black,width: 1),
                    borderRadius: const BorderRadius.all(Radius.circular(10.0)),
                  ),
                  child: DropdownButtonHideUnderline(
                    child: DropdownButton<LeaveType>(
                      isExpanded: true,
                      //initial value 
                      value: selectedLeave != null ? items[0] : null,
                      icon: const Icon(Icons.arrow_downward),
                      iconSize: 24,
                      elevation: 16,
                      hint: text("Leave Type"),
                      style: const TextStyle(
                          color: Colors.black
                      ),
                      onChanged: (LeaveType  value) {
                        setCustomState(() {
                          selectedLeave = value;
                          items.remove(selectedLeave);
                          items.insert(0, selectedLeave);
                        });
                      },
                      items: items
                          .map((leave) {
                        return  new DropdownMenuItem<LeaveType>(
                          value: leave,
                          child: text(leave.name),
                        );
                      }).toList(),
                    ),
                  ),
                ),
              ),
            ]
        );
  }

I changed as below and it got solved:

Initial Code:

List<GamesModel> users = <GamesModel>[
  new GamesModel(1,"Option1"),
  new GamesModel(2,"Option2"),

];
return users;

Changed Code:

List<GamesModel> users = <GamesModel>[
      const GamesModel(1,"Option1"),
      const GamesModel(2,"Option2"),
];
return users;

If anybody want i can put the whole code

          DropdownButton<String>(
            iconEnabledColor: Colors.cyan.withOpacity(.6),
            isExpanded: true,
            itemHeight: 50,
            iconSize: 30,
            hint: Text("Choose Province"),
            items: _provinces
                .map((e) => DropdownMenuItem(
              child: Text(e),
              value: e,
            ))
                .toList(),
            value: _Province,
            onChanged: (String? value) async{
              final respnose=await FirebaseFirestore.instance.collection('city').where('provinceName',isEqualTo: value).get();
              _city=[];
              for(var item in respnose.docs){
                print(item.data());
                _city.add(item.data()['name']);
              }
              
              print(_Province);
              setState(() {
                _city=_city;
                _Province = value;
              });
            },
          ),
          SizedBox(height: 20,),

          DropdownButton<String>(
            iconEnabledColor: Colors.cyan.withOpacity(.6),
            isExpanded: true,
            itemHeight: 50,
            iconSize: 30,
            hint: Text("Choose City"),
            items:_city
                .map((e) => DropdownMenuItem(
              child: Text(e),
              value: e,
            ))
                .toList(),
            value: _City,
            onChanged: (String? value) async{
              setState(() {
                _town=[];
                _Town=null;
              });
              print(_town);
              final respnose=await FirebaseFirestore.instance.collection('town').where('cityName',isEqualTo: value).get();
              print(respnose.docs);


              for(var item in respnose.docs){
                print(item.data());
                _town.add(item.data()['name']);
              }
              print(_town);
              print(_City);
              setState(() {

                _City = value;
                _town=_town;
              });
            },
          ),
          SizedBox(height: 20,),
          
          if(true)
          DropdownButton<String>(
            iconEnabledColor: Colors.cyan.withOpacity(.6),
            isExpanded: true,
            itemHeight: 50,
            iconSize: 30,
            hint: Text("Choose Town"),
            items:_town
                .map((e) => DropdownMenuItem(
              child: Text(e),
              value: e,
            )
            )
                .toList(),
            value: _Town,
            onChanged: (String? value)async {
              print(_Town);
              setState(() {
                _Town = value;
              });

         

This error also occurs if you forget to give dropdown menu items a value. ==== WORKS ====

<String>['A', 'B', 'C'].map<DropdownMenuItem<String>>((vehicle) {
        print("vehicle is $vehicle");
        print("vehicle is equal ${vehicle == x.value}");
        return DropdownMenuItem<String>(
          value: vehicle,
          child: Text(
            // vehicle.vehicleInfo!.vehicleType!,
            vehicle,
            style: TextStyle(
              color: Colors.grey[600],
            ),
          ),
        );
      }).toList(),

==== DOESNT WORK ====

<String>['A', 'B', 'C'].map<DropdownMenuItem<String>>((vehicle) {
        return DropdownMenuItem<String>(
          child: Text(
            vehicle,
            style: TextStyle(
              color: Colors.grey[600],
            ),
          ),
        );
      }).toList(),

I had the same problem, and the solution is to fill the value of DropdownButton(value: (use a value from the items you set) you can not use any value you want, but it should be one of the items that you set for the DropdownMenuItem.

Note that if the list has duplicated values, it will also has this error. For example, if languages = ["English", "English", "French"];

then if I set the default language = "English".

      DropdownButton<String>(
        value: language,
        icon: const Icon(Icons.arrow_downward),
        iconSize: 24,
        elevation: 16,
        style: const TextStyle(color: AppColors.highLightTextColor),
        underline: Container(
          height: 1,
          color: AppColors.underLineColor,
        ),
        onChanged: (String? newValue) async {
          setState(() {
            language = newValue;
          });
        },
        items: languages.map<DropdownMenuItem<String>>((String value) {
          return DropdownMenuItem<String>(
            value: value,
            child: Text(value),
          );
        }).toList(),
      ),

Remove the duplicate values, then it works.

I think because of the update in the framework, the error came out

Here is how you can solve it

DropdownButton(
              hint: const Text("Please choose your gender"),
              items: <String>["Male", "Female", "Rather not say"]
                  .map<DropdownMenuItem<String>>((e) {
                return DropdownMenuItem<String>(
                    value: e, child: Text(e.toString()));
              }).toList(),
              onChanged: (String? value) {
                setState(() {
                  dropdownValue = value!;
                });
              });

Note that: dropdownValue is a string variable defined at the top

In my case, i use empty String for default

value : dropdownValue != "" ? dropdownValue : null

Like this, errors be gone

make sure you have pass the value perameter to dropdownmenuitem

DropownMenuItem(
   vale:"unique val",
   child:SizedBox()
)