How to pass a single object[] to a params object[]

A simple typecast will ensure the compiler knows what you mean in this case.

Foo((object)new object[]{ (object)"1", (object)"2" }));

As an array is a subtype of object, this all works out. Bit of an odd solution though, I'll agree.

This is a one line solution involving LINQ.

var elements = new String[] { "1", "2", "3" };
Foo(elements.Cast<object>().ToArray())

The params parameter modifier gives callers a shortcut syntax for passing multiple arguments to a method. There are two ways to call a method with a params parameter:

1) Calling with an array of the parameter type, in which case the params keyword has no effect and the array is passed directly to the method:

object[] array = new[] { "1", "2" };

// Foo receives the 'array' argument directly.
Foo( array );

2) Or, calling with an extended list of arguments, in which case the compiler will automatically wrap the list of arguments in a temporary array and pass that to the method:

// Foo receives a temporary array containing the list of arguments.
Foo( "1", "2" );

// This is equivalent to:
object[] temp = new[] { "1", "2" );
Foo( temp );

In order to pass in an object array to a method with a "params object[]" parameter, you can either:

1) Create a wrapper array manually and pass that directly to the method, as mentioned by lassevk:

Foo( new object[] { array } );  // Equivalent to calling convention 1.

2) Or, cast the argument to object, as mentioned by Adam, in which case the compiler will create the wrapper array for you:

Foo( (object)array );  // Equivalent to calling convention 2.

However, if the goal of the method is to process multiple object arrays, it may be easier to declare it with an explicit "params object[][]" parameter. This would allow you to pass multiple arrays as arguments:

void Foo( params object[][] arrays ) {
  foreach( object[] array in arrays ) {
    // process array
  }
}

...
Foo( new[] { "1", "2" }, new[] { "3", "4" } );

// Equivalent to:
object[][] arrays = new[] {
  new[] { "1", "2" },
  new[] { "3", "4" }
};
Foo( arrays );

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Edit: Raymond Chen describes this behavior and how it relates to the C# specification in a new post.