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I'm looking to set a leader from an array of sprites, I keep on getting a NullReferenceException was unhandled error from within my main game class when calling the UpdateMouse Method. What have I dont wrong here?

class MouseSprite

    Random random = new Random();
    private MouseSprite leader;

    public void UpdateBoundaryBox()
        mouseBounds.X = (int)mousePosition.X - mouseTexture.Width / 2; mouseBounds.Y = (int)mousePosition.Y - mouseTexture.Height / 2;

    public void UpdateMouse(Vector2 position, MouseSprite [] mice, int numberMice, int index)

        Vector2 catPosition = position;
        int enemies = numberMice;
        this.alive = true;

        mice[random.Next(0, mice.Length)] = leader;
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You will benefit greatly from this post: How to use a debugger. –  Byte56 Dec 8 '12 at 16:36
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closed as too localized by Byte56, Trevor Powell, Josh Petrie, Laurent Couvidou, bummzack Dec 16 '12 at 13:08

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2 Answers

up vote 0 down vote accepted

leader is uninitialized. I assume you're trying to pick a random leader, not set the leader to some mouse randomly (overwriting the mouse stored there with null). You want to swap your assignment statement.

leader = mice[random.Next(0, mice.Length)];
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random value is calculated right... it returns a number between 0 and mice.Lenght-1, remmeber that min value is inclusive and max value is exclusive... msdn.microsoft.com/en-us/library/2dx6wyd4.aspx –  Blau Dec 8 '12 at 16:33
Ah, true enough, I'm used to inclusive randoms. –  Byte56 Dec 8 '12 at 16:34
even swapping them around I still get the same error –  Craig Dec 8 '12 at 16:46
sorry it did fix it, thanks! –  Craig Dec 8 '12 at 16:52
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Null reference exceptions are thrown because you are trying to access to a reference typed variable that is not assigned.

In the UpdateMouse method code, the only candidate to throw that exception is the array, that maybe is not assigned, check it.

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This is a symptom, not a solution. While it may be obvious to you or I how to fix this, this answer should still include a solution. As it stands, it's just a suggestion on what's wrong, instead of how to fix it. So more of a comment. –  Byte56 Dec 10 '12 at 19:12
"Check it" is not a solution... –  Byte56 Dec 10 '12 at 23:45
:P ... the question is "What have I dont wrong here?" Mine is a simple answer that complements yours. If you see the code and where is throwed the exception... is the more reasonable... your answer is more heuristic and other approach to solution... is funny that I could downvote you by an objetive fail in your answer... and you downvote me by subjetive criteria... –  Blau Dec 11 '12 at 9:27
This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. –  Laurent Couvidou Dec 11 '12 at 10:08
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