Cannot use string as an type-specific constraints

I am a starter in Web API. I try to implement GET service and use string as a constraint. VS throw me the error that.

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Server Error in '/' Application.

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The inline constraint resolver of type 'DefaultInlineConstraintResolver' was unable to resolve the following inline constraint: 'string'.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.InvalidOperationException: The inline constraint resolver of type 'DefaultInlineConstraintResolver' was unable to resolve the following inline constraint: 'string'.

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I found this from StackOver flow. The string cannot be applied to type-specific constraints. Now, I get it that why to use the POST when developer need to parse the parameter instead of GET.

// Type-specific constraints
{ "bool", typeof(BoolRouteConstraint) },
{ "datetime", typeof(DateTimeRouteConstraint) },
{ "decimal", typeof(DecimalRouteConstraint) },
{ "double", typeof(DoubleRouteConstraint) },
{ "float", typeof(FloatRouteConstraint) },
{ "guid", typeof(GuidRouteConstraint) },
{ "int", typeof(IntRouteConstraint) },
{ "long", typeof(LongRouteConstraint) },

// Length constraints
{ "minlength", typeof(MinLengthRouteConstraint) },
{ "maxlength", typeof(MaxLengthRouteConstraint) },
{ "length", typeof(LengthRouteConstraint) },

// Min/Max value constraints
{ "min", typeof(MinRouteConstraint) },
{ "max", typeof(MaxRouteConstraint) },
{ "range", typeof(RangeRouteConstraint) },

// Regex-based constraints
{ "alpha", typeof(AlphaRouteConstraint) },
{ "regex", typeof(RegexRouteConstraint) }