AllAccepted answers
Eduard Lebedyuk · Aug 7, 2019

Here's a way without indirection:

Class MyPackage.MyClass
{

ClassMethod MyMethod(p1 = 1, p2 = 2, p3 = 3)
{
    Write p1,"-",p2,"-",p3,!
}

/// do ##class(MyPackage.MyClass).Test()
ClassMethod Test()
{
    Do ..FromJson({})
    Do ..FromJson({"p1":"first value", "p2":"second value"})
    Do ..FromJson({"p1":"first value", "p3":"third value"})
    Do ..FromJson({"p2":"second value"})
}

/// do ##class(MyPackage.MyClass).ArgPosMapping("MyPackage.MyClass", "MyMethod")
ClassMethod ArgPosMapping(class, method, Output map)
{
    kill map
    set formalspec = $$$comMemberKeyGet(class,$$$cCLASSmethod,method,$$$cMETHformalspecparsed)
    for i=1:1:$ll(formalspec) {
        set arg = $lg(formalspec, i)
        set map($lg(arg, 1)) =  i
    }
    
    set map = $ll(formalspec)
}

ClassMethod FromJson(json)
{
    Do ..ArgPosMapping($classname(), "MyMethod", .map)
    Set iterator = json.%GetIterator()
    Set position = 0
    While iterator.%GetNext(.key, .value) {
        Set position = position + 1
        Set arguments(map(key)) = value
    }
    
    Set arguments = map

    Do ..MyMethod(arguments...)
}

}
Eduard Lebedyuk · Jul 27, 2019

Tip.

Metadata does not change from one row to the other and accessing it takes time, so removing it from the row loop would improve the execution speed:

Class User.Test
{

Query TestQuery() As %SQLQuery
{
SELECT * FROM Sample.Person
}

/// do ##class(User.Test).Test()
ClassMethod Test()
{
    for method = "EveryRow", "Once" {
        set rSet = ..TestQueryFunc()
        
        set start = $zh
        do $classmethod(, method, rSet)
        set end = $zh
        
        write $$$FormatText("Method %1 took %2 sec.", method, end - start), !
    }
}

ClassMethod EveryRow(rSet As %SQL.ISelectResult)
{
    
    set tResults = []
    while rSet.%Next() {
        set tRow = {}
        set tMetadata = rSet.%GetMetadata()
        set tColumnCount = tMetadata.columns.Count()
        for x=1:1:tColumnCount {
            set tColumn = tMetadata.columns.GetAt(x)
            set tColumnName = tColumn.colName
            //do tRow.%Set(tColumnName, rSet.%GetData(x) )
            set $PROPERTY(tRow,tColumnName) = $PROPERTY(rSet,tColumnName)
        }
        do tResults.%Push(tRow)
    }
}

ClassMethod Once(rSet As %SQL.ISelectResult)
{
    set tResults = []
    set tColumns = ""
    
    set tMetadata = rSet.%GetMetadata()
    set tColumnCount = tMetadata.columns.Count()
    for x=1:1:tColumnCount {
        set tColumn = tMetadata.columns.GetAt(x)
        set tColumnName = tColumn.colName
        set tColumns = tColumns _ $lb(tColumnName)
    }

    while rSet.%Next() {
        set tRow = {}
        for x=1:1:tColumnCount {
            do tRow.%Set($lg(tColumns, x), rSet.%GetData(x) )
        }
        do tResults.%Push(tRow)
    }
}

}

Results for me:

>do ##class(User.Test).Test()
Method EveryRow took .017803 sec.
Method Once took .01076 sec.