Show hide text box when specific option is selected from dropdown inside dynamic form Angular

Rajnikant :

I have a form where a user could add one/more div of Address on click of add button.

I want if user select options=5 from the dropdown, want to show and hide textbox in that particular address Div.

Component Code

        get contactFormGroup() {
            return this.form.get('Array') as FormArray;
          }

          ngOnInit() {
            this.form= this.fb.group({
              Array: this.fb.array([])
            });
          }

          createContact(): FormGroup {
            return this.fb.group({
              ABC: [null, Validators.compose([Validators.required])],
              Test: [null, Validators.compose([Validators.required])]
            });
          }

          addContact() {
            this.Group.push(this.createContact());
          }

          showValue(event) {
            const selectedValue = event;
            if (selectedValue === '5') {
                this.showValuetxtbox = true;
            } else {
                this.showValuetxtbox = false;
            }
          }

JMP :

As you are looping to add the divs, you could use a template reference variable on the drop down. e.g #select then refer to that in the *ngIf:

    <form [formGroup]="addExpressionform">
        <div formArrayName="expressionArray">
            <div *ngFor="let item of contactFormGroup.controls; let i = index;" [formGroupName]="i">

              <carbon-dropdown #select
                                (optionSelected)="showValue($event)"
                                [formControlN]="'UOM'"
                                [options]="listOptions" [formGroup]="item"
                                name="UOM" 
                                >
                            </carbon-dropdown>

                            <carbon-text-input *ngIf="select.value == 5" 
                                [formControlN]="'Value'"
                                [formGroup]="item"
                                name="Value"
                                >
                            </carbon-text-input>
                    <carbon-button type="primary" (click)="submit()" id="save-parameter">Save</carbon-button>

            </div>                  
        </div>
    </form>

Simplified StackBlitz demo.