I am calling a function which takes the same form as string.format where the first parameter is a string and the remainder are the replacement values. I have the string in a variable and the replacement values in an array, how can I call this function given any number of objects in the array? Simply passing in the array as the last argument does not work.
Use the params keyword:
public string MyMethod(string value, params object[] args)
{
// as an example
return string.Format(value, args);
}
Then you can call it either with individual values
MyMethod("Test", "value1", "value2");
Or with an array
MyMethod("Test", new [] { "value1", "value2" });
you need to use the params keyword
The function I was calling had the signature
public static IQueryable Where(this IQueryable source, string predicate, params object[] values)
Thhe issue was that I was passing in an array of ints as the last parameter. When I createed a new array of objects poplulated from that initial array and passed it in it worked. Thanks for the answers
Related
Why can I invoke Parse method without parentheses since that method has 4 overloads?
For example in this case:
string[] aTemp = Console.ReadLine().Split(' ');
int[] a = Array.ConvertAll(aTemp, int.Parse);
The signature for ConvertAll is actually this:
public static TOutput[] ConvertAll<TInput, TOutput>(
TInput[] array,
Converter<TInput, TOutput> converter
)
Which the compiler can infer to be:
public static int[] ConvertAll<string, int>(
string[] array,
Converter<string, int> converter
)
From the signature for Int32.Parse:
public static int Parse(
string s
)
If you wanted to write out in long hand:
Converter<string, int> converter = new Converter<string, int>(Int32.Parse);
string[] aTemp = Console.ReadLine().Split(' ');
int[] a = Array.ConvertAll<string, int>(aTemp, converter);
Note: Converter<TInput, TOutput> is actually a delegate that takes as input a parameter of type TInput and returns a value of type TOutput.
Array.ConvertAll takes two parameters, an array of TInput, and a converter delegate from TInput to TOutput. There's only one overload of int.Parse that matches the signature of a converter delegate -
public static int Parse(
string s
)
Putting together all of the information available, we can pick the right method to call.
As I was experimenting with params, I noticed the MS documentation says if you pass an array of int as a method parameter that has a signature of params object[] myParam, it will become a multi-dimensional array. But I noticed if you pass an array of objects[] or strings[] it does not. This seems like it would be a headache to work with, as you have to check for multi-dim arrays.
See MSDN link
Example:
public static void UsingParams(params object[] myParam)
{
//Code to return myParam
}
//myParam[0][0] = {1, 2}, multi-dimensional
int[] myIntArray = {1, 2};
UsingParams(myIntArray);
//myParam[0] = {"1", "2"}, single-dimensional
string[] myStrArray = {"1", "2"};
UsingParams(myStrArray);
Why does this occur?
Whenever you have a params parameter the compiler will attempt to accept an array representing all of the values for the params argument, if the parameter at the position in question is valid in that context. If it's not, then it tries to treat it as one item in the array of params values, rather than as the whole array. If it can't do that either then it will fail to compile.
In your second example a string[] can be implicitly converted to an object[], so it is passed as the entire list of parameters. This implicit conversion is valid because of array covariance.
In your first example the int[] cannot be implicitly converted to an object[] (array covariance is limited to reference types), so it is treated as one value in the array. An int[] can be implicitly converted to object, so what is passed is an object array containing an int[] as its only item. Note that an array with another array as an item is dramatically different from a multi-dimensional array.
C# is trying to figure out, when you only pass one value to a params argument, whether you mean for that value to be the array represented by the argument, or whether you're passing it the first argument of a larger array.
If you remove the params keyword, you'll see that int[] cannot be converted directly into an object[] (due to int being a non-reference type):
So C# figures it must just be the first of your params that you're passing in, rather than the entire array. It converts your code to this:
int[] myIntArray = {1, 2};
UsingParams(new object[]{myIntArray});
Basically your method signature is taking one or more objects and consolidating them into an array called myParam.
If multiple objects are passed individually, such as the call UsingParams(1, "hello", ...), they'll automatically be converted to the object[] array. This is a compiler trick / syntactic sugar.
If the object being passed is not an object[] array or list of individual objects, it will become the first argument of your array. In other words, if you pass an int[] then your myParam will be an array, the first element of which is an array also, making it a jagged array. This is because int[] is an object and the compiler isn't smart enough to figure out what you're doing and makes it the sole element of the object[] array. There is no implicit cast from int[] to object[] and this is why it doesn't happen.
The only time you can pass an array that will be populated as you'd expect is when that array type is object[], such as new object[] { 1, "hello", ... } or the array type is covariant. In this case, a string[] array is covariant and can be implicitly cast to an object[] while an int[] cannot.
In summary:
UsingParams(1, "hello") = good
UsingParams(new object[] { 1, "hello" }) = good
UsingParams(new string[] { "hi", "hello" }) = good (due to array covariance)
UsingParams(new int[] { 1, 2 }) = bad (no array covariance), will be a jagged array
Further reading on array covariance rules which also cites: "Array covariance specifically does not extend to arrays of value-types. For example, no conversion exists that permits an int[] to be treated as an object[]."
This question already has answers here:
Calling a function using reflection that has a "params" parameter (MethodBase)
(3 answers)
Closed 8 years ago.
Trying to invoke a method with an arbitrary number of parameters:
Type objType = obj.GetType();
MethodInfo method = objType.GetMethod("InvokedMethod");
method.Invoke(obj, new string[] { "param1", "param2" });
The method signature looks like that:
public void InvokedMethod(params string[] args) { ... }
Why I get following Exception:
System.Reflection.TargetParameterCountException (Parameter count mismatch)
The method doesn't accept two parameters, it accepts one parameter that is an array. The compiler will perform a transformation such that a method call of two strings will be transformed into a call of a single array with two values. That transformation won't be done for you when using reflection. You'll need to explicitly create an array and put the two values in it:
method.Invoke(obj, new object[] { new[]{"param1", "param2"} });
Remember that Invoke doesn't accept a single value of that one parameter either. It accepts an array of all parameters. Passing new string[] { "param1", "param2" } to Invoke is telling Invoke that you have two parameters, each of which are strings. You need to wrap your one array parameter in another array so that Invoke sees that you have one parameter that is itself an array.
Try this:
method.Invoke(obj, new object[] {new string[] {"param1", "param2"}});
.Invoke takes an array of arguments that represents the entire signature. in your case the first argument should be an array of strings. So you have to pass in an array of objects with the first element being the array of strings.
I am doing a simple program in Silverlight to inkove javascript function in silverlight.
The silverlight function is as under
void InvokeJS(params object[] items)
{
object result = System.Windows.Browser.HtmlPage.Window.Invoke("JSFunction", items);
}
Pasing value to this function is happening as under
InvokeJS((object)new object[]{ (object)"10", (object)"20"})
And the JS function is as under
function JSFunction(params) {
alert(params);
}
Now how to read the params value in javascript?
The params variable is just the first of many arguments being passed in. You can access the other arguments using the following syntax:
alert(this.arguments[0]);
alert(this.arguments[1]);
alert(this.arguments[2]);
If you're passing all the arguments in a single variable, it will be an array so use:
alert(params[0]);
alert(params[1]);
alert(params[2]);
To the called function, the params array is just that, an array.
In this case you will have an array that looks like this:
[ [ "10", "20" ] ]
I got it
alert(params[0]); alert(params[1]);
I'm trying to call String.Format from with in a Linq.Expression tree. Here's a quick example:
var format = Expression.Constant("({0}) {1}");
var company = Expression.Property(input, membernames.First());
var project = Expression.Property(input, membernames.Last());
var args = new Expression[] {format, company, project};
var invoke = Expression.Call(method,args);
The issue however is that String.Format has the signature of:
String.Format(string format, params object[] args)
and I'm trying to pass in Expression[].
Now I could go through all the trouble of creating an array, populating it with the results of my expressions, but what I really want the result to be, is something like this:
String.Format("({0}) {1}", input.foo, input.bar)
How do I go about calling a params function via Linq Expressions?
What params actually does is just to specify ParamArrayAttribute for that parameter. The C# compiler understands this, and creates the array behind the scenes.
Expressions don't understand this, so you actually have to create the array by yourself, if you want to call a method with params. This can be also seen by the fact that when you assign a lambda using params-method to an expression, the expression contains the array creation:
Expression<Func<string>> expression = () => string.Format("",1,2,3,4);
string expressionString = expression.ToString();
Here, expressionString will contain this string:
() => Format("", new [] {Convert(1), Convert(2), Convert(3), Convert(4)})
To create an expression that creates an array, use the Expression.NewArrayInit() method.
That being said, if you only want two parameters (or one or three), there is an overload of string.Format() that you can use directly from an expression.
params is just syntactic sugar. Ultimately the parameter is just an array. Therefore, the parameter type should be object[] and an expression describing such an array is what you should pass as the second argument. In other words, you should only have two arguments, not three. And the second argument should be a two-element array containing what is currently your 2nd and 3rd arguments.