Dart : How to Check if a variable type is String

Instead of

if(T is String)

it should be

if(_val is String)

The is operator is used as a type-guard at run-time which operates on identifiers (variables). In these cases, compilers, for the most part, will tell you something along the lines of T refers to a type but is being used as a value.

Note that because you have a type-guard (i.e. if(_val is String)) the compiler already knows that the type of _val could only ever be String inside your if-block.

Should you need to explicit casting you can have a look at the as operator in Dart, https://www.dartlang.org/guides/language/language-tour#type-test-operators.

Use .runtimeType to get the type:

void main() {

var data_types = ["string", 123, 12.031, [], {} ];`
  
for(var i=0; i<data_types.length; i++){

    print(data_types[i].runtimeType);

    if (data_types[i].runtimeType == String){
      print("-it's a String");

    }else if (data_types[i].runtimeType == int){
      print("-it's a Int");
      
    }else if (data_types[i].runtimeType == [].runtimeType){
      print("-it's a List/Array");

    }else if (data_types[i].runtimeType == {}.runtimeType){
      print("-it's a Map/Object/Dict");

    }else {
      print("\n>> See this type is not their .\n");
    } 
}}