How to set body of the property 'public string this[string key] { get; set; }'

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C#

ASP.NET

I declare a property. code is below:

public string this[string key] { get; set; }

It shows me error: " must declare a body because it is not marked abstract, extern, or partial "

Then i changed the property to:

private string[] _key;
public string this[string key] { get { return _key; } set { _key = value; } }

Not it showing me error: " Cannot implicitly convert type 'string[]' to 'string' "
and " Cannot implicitly convert type 'string' to 'string[]' "

Where is the problem? May be I'm setting the body in wrong way.
Please, tell me the way how to set this type of property.

Thanks in advance...